It's quite clear from Buffy and Angel that vampires with souls are vanishingly rare. The Shanshu Prophecy refers to the vampire with a soul, and the appearance of Spike as a second such causes great consternation. (Beyond the general consternation Spike commonly causes.) Now, Angel was first re-ensouled forcibly by a gypsy tribe when he ate their favorite child. He's since been re-ensouled twice more by Willow. Apparently a different set of gypsies tried the ritual on a different vampire, and failed.
Is any explanation ever provided as to why this ritual is not at least attempted any other times? Forcibly re-ensouling a vampire against his will should greatly reduce its threat potential, as was seen in Angelus/Angel. And it can apparently be done remotely! What a great weapon, if you can pull it off. And Willow did it with almost zero magical training. Why aren't Slayers and allies casting this all the time on every standing vampire threat?