The charm only worked on the Dark Lord and maybe his operatives.
The protective charm on the Dursleys’ house kept Harry safe from the Dark Lord and, possibly but not necessarily, those working on his behalf as well. It didn’t protect him from all harm, or even all magical harm.
“While you can still call home the place where your mother’s blood dwells, there you cannot be touched or harmed by Voldemort. He shed her blood, but it lives on in you and her sister. Her blood became your refuge. You need return there only once a year, but as long as you can still call it home, whilst you are there he cannot hurt you.”
- Harry Potter and the Order of the Phoenix, Chapter 37 (The Lost Prophecy)
However, Umbridge was the one who sent the Dementors. Although she was indeed trying to harm Harry, she certainly wasn't the Dark Lord himself ,and wasn’t connected to the Dark Lord at all. She might have shared views with him on wizards born to Muggles, but she wasn’t actually working for him. She sent them independently.
“He never knew I ordered Dementors to go after Potter last summer, but he was delighted to be given the chance to expel him, all the same.’
‘It was you?’ gasped Harry. ‘You sent the Dementors after me?’
‘Somebody had to act,’ breathed Umbridge, as her wand came to rest pointing directly at Harry’s forehead. ‘They were all bleating about silencing you somehow – discrediting you – but I was the one who actually did something about it … only you wriggled out of that one, didn’t you, Potter? Not today though, not now –”
- Harry Potter and the Order of the Phoenix, Chapter 32 (Out of the Fire)
Therefore, even if it would have worked on Dementors sent by the Dark Lord, it wouldn’t have worked in the case of Umbridge sending them.
Also, the Dementors were on Magnolia Road, not Privet Drive - which may be out of range of the charm even for the Dark Lord. While this would be a walkable distance from the Dursleys’ house, since Harry walked there, it wasn’t right near the house. It’s unclear what the range of the Charm is, and at what point the Dark Lord himself would be able to harm Harry.