Painting over filled woodwork

Hi all,

Having some problems with paint crazing and flaking off woodwork where I've filled cracks, holes etc on several different areas of woodwork. Here's what I've done so far

1 - Stripped and sanded a door, filled holes with Screwfix no nonsense wood filler (71961), used B&Q own acrylic primer/undercoat, then B&Q own water based satin top coat. A day or two later all the areas I'd filled with crazed or completely lifted in rough little flakes, looked almost like a cheese grater.

2 - Next we did a window frame. This time i decided to prime first, so used Leyland Acrylic Primer/Undercoat, then filled (this time some Wickes ready mix) then top coat of the same B&Q satin stuff. This also crazed, although not as bad.

We got the window frame looking good by sanding back the crazed areas then coat of Zinsser oil based primer, then top coat of the water based satin again.

So my question is basically how should you fill and paint bare wood? Neither Fill-Prime-Topcoat nor Prime-Fill-Topcoat seemed to work for me, so what am I missing? Should I seal the filler with something (mist coat, oil based primer..?) before over painting?

Thanks!