Paint peeling from filled areas

I recently repainted the walls of my living room but I'm having trouble with the new paint coming off in areas where there used to be a freeze.
Before repainting I removed the freeze and in places some of the paint below came with it, exposing bare plasterboard below. I assume the wallpaper paste used to fix the freeze loosened the paint, leading to it coming off when I removed the freeze.
In the areas where the paint had come away, I knocked off any surrounding loose paint, filled the area with polyfilla, sanded it smooth and then painted. It's these areas that I am now losing the new paint; it cracks and just peels off the wall. The polyfilla stays on the wall. The areas were fully dry before painting.
What can I do to remedy this? I've read about sealing plasterboard with a watered-down PVA mix; is that appropriate? Alternatively, are there any fillers designed to be directly applied to plasterboard? The affected areas are quite small, no more than a square foot or two in total.
Thanks for any help.
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