Vassals don't seem to be coring provinces and/or use idiotic taxing.

[Trashcycling 17]

Rebels keep getting occupying my vassals' land.  I gave transylvania bits of moldova and these guys just won't stop appearing. Hope this gets fixed soon.

[HaSa 118]

same tbh, I vassalised wales and give it some of Ireland and it would get revolts every year because it would not core, but maybe they were just broke and couldn't afford to core who knows

[Łukasz Jakowski 13,394]

Gift them gold

[Ilovethisgame2 813]

They're just unable to afford it. Keep gifting them gold until they core the provinces, sometimes they just invest somewhere else or they're in debt idk.

[Lazer 10]

In vassal menu check how much money they have to give you, if you have this setting set on high then im not surprised.