Considering the battle the petty-realms had with the Witch-king lasted for hundreds of years, one would think Gondor and Rivendell would act quicker, surely?
So why is it not until Arnor is pretty much lost that Rivendell and Gondor finally dispatch Glorfindel and Eärnur to aid Arnor?
They were stretched thin:
Eärnil was a wise man, and not arrogant, even if, as to most men in Gondor, the realm in Arthedain seemed a small thing, for all the lineage of its lords.
He sent messages to Arvedui announcing that he received the crown of Gondor, according to the laws and the needs of the South-kingdom, "but I do not forget the loyalty of Arnor, nor deny our kinship, nor wish that the realms of Elendil should be estranged. I will send to your aid when you have need, so far as I am able."
It was, however, long before Eärnil felt himself sufficiently secure to do as he promised.
Return of the King Appendix A "Annals of the Kings and Rulers" I "The Númenórean Kings" (iv) Gondor and the Heirs of Anárion
Gondor is just coming off of a bad century, with the Wainrider invasion right on the heels of a massive plague. Eärnil made his promise in T.A. 1945, shortly after he finally defeated the Wainriders, and Gondor wasn't ready to commit to a major operation.
This isn't clear from the text. It's entirely possible that the Elves didn't know the war was going so poorly until Eärnur arrived at the Grey Havens:
[A]t last in the autumn of 1973 messages came to Gondor that Arthedain was in great straits, and that the Witch-king was preparing a last stroke against it. Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he could, and with as great strength as he could spare. Too late. Before Eärnur reached the havens of Lindon, the Witch-king had conquered Arthedain and Arvedui had perished.
Return of the King Appendix A "Annals of the Kings and Rulers" (iv) Gondor and the Heirs of Anárion
But we can't be sure.
The Elven response was more from Lindon than Rivendell, there were not enough Elves in Rivendell to provide much assistance in a military campaign. Arnor/Arthedain and Gondor had not had regular contact in some time either, from the Tale of Years:
1940: Gondor and Arnor renew communications and form an alliance. Arvedui weds Firiel daughter of Ondoher of Gondor.
Gondor had its own troubles with the Kin-Strife and the Easterlings and Southrons over these years as well.
Before Eärnil II took the throne in T.A. 1945 "Gondor came near to destruction." ("Gondor and the Heirs of Anárion") The Wainriders sat on their haunches after defeating the Northern Army. Eärnil who was a cousin to the King and led the Southern army came up from the South after victory there and took the Wainriders out. It was only 4 years after Arnor and Gondor started to communicate again that King Ondoher died (1944) in this war along with his two sons. It is said that the reason that the North and South kingdoms made contact (1940) with one another again was because
they perceived that some single power and will was directing the assault from many quarters upon the survivors of Numenor.
("Gondor and the Heirs of Anárion")
Eärnil was never quite secure in sending men to aid Arthedain for quite some time after becoming king [around 30 years] and when he finally did send help after receiving dire messages from the North he could only send a a small force. However, even at this point in Gondor's waning they have a clear advantage over the North in strength.
Then Eärnil sent his son Eärnur north with a fleet, as swiftly as he could, and with as great strength as he could spare. Too late. [...]
But when Eärnur came to the Grey Havens there was joy and great wonder among both Elves and Men. So great in draught and so many were his ships that they could scarcely find harbourage, though both the Harlond and the Forlond were also filled; and from them descended an army of power, with munition and provision for a great war of kings. Or so it seemed to the people of the North, though this was but a small sending-force of the whole might of Gondor.
(Appendix A., "Gondor and the heirs of Anárion" - emphasis added)
As to the Elves, Arthedain in particular remained friendly with them and were receiving help from Lindon and Rivendell for their long war with Angmar.
Amlaith and his descendants maintained friendship with the Eldar, especially with Círdan at the Havens.
("The Heirs of Elendil," Chapter 7)
When the other two kingdoms are ravaged Arthedain holds out with "aid from Lindon and Imladris". Before Angmar invaded Arnor in 1409 Rivendell had been besieged, and during that invasion Weathertop was destroyed. The last Prince of Cardolan (such as the one in Dol Amroth) was killed when this invasion came. The Dúnedain held out at Fornost with aid from Lindon.
Araphor son of Arveleg was not yet full-grown, but he was valiant, and with aid from Círdan he repelled the enemy from Fornost and the North Downs.
(Appendix A: "The North-kingdom and the Dúnedain")
For the most part the Dúnedain held out at Fornost, the North Downs, and at Tyrn Gorthad (those fighting here were without their Prince who had died but was said to have been buried here, "some say the mound in which the Ringbearer was imprisoned had been the grave of the last Prince of Cardolan" - "The North-kingdom and the Dúnedain").
So the North Kingdom did get help from Lindon and Rivendell. Elrond even sent for Elves to help from Lórien, but clearly it did not do much good. For a lot of this time the 3 separate Kingdoms in Arnor were fighting Angmar. For about 300 years before Arthedain fell it was really Arthedain versus Angmar. The Dúnedain in Cardolan were wiped out finally in the plague in 1636 and Rhudaur had long fallen before the Plague to Angmar.
Basically it was slow decay and constant picking away at both kingdoms in decline. Both kingdoms aided in their decline with in-fighting, the split in the North, the Kin-strife in the South and a further diminishment of the royal line in the South with members of that line renouncing their inheritance and joining rebels since they were fearful of the jealous kings.
