Painting previously painted walls

[tumchy]

Hi, new here and looking for advice on painting walls.
I had them skimmed a few years ago then put up wallpaper. I stripped the wallpaper and gave it a light sanding.
Today I thought all I had to do was give it 2 coats of matt emulsion, but after 1 coat, the some of the paint has cracks and rough bits. After reading posts on here, i think it must be wallpaper paste residue.
Would be grateful for advice.

 

[joe-90]

Don't paint over wallpaper paste.

 

[tumchy]

Too late, I thought the wall was free of paste when I painted it. Just wondering what's best to do now.....will it be ok to sand down and paint again?

 

[tumchy]

I sanded it down, washed it with sugar soap, let it dry and gave it another coat of paint (Dulux matt) There's still a couple of bits cracking.
Can anybody tell me what to do???