What's caused this?

[Steve]

Paint flaking off a door frame. it is b&q satin paint, Painted one day prior to this photo. The wood is old but was sanded prior to painting. Sanding was awkward due to the profile of the wood though.
Now I think about it the plasterer did get very messy with his pva which has made some of the walls blotchy and hard to paint. could this be pva? Or is it something else?

 

[chappers]

was it back to bare wood, did you prime or undercoat.

 

[Steve]

Parts of it were bare wood yes. only little bits where the paint peeled off when we removed the old architraves. I know this will never be a perfect finish but I guess I should have primed and undercoated these bits before I painted it. only other option is to strip all the paint off the frame which isn't an option right now.