Paint cracking on wooden interior door...help!

[Skifreak]

I had a door "diped n stripped" then once it had dried out put knotting solution on and have now used primer/undercoat.

The problem I have is that in certain places on the door there is a yellow/brown shadow and paint is cracking, it's almost like these parts of the door have dried out too much. I have tried sanding and applying more coats but the problem only seems to be getting worse.

Any suggestions please as I really don't know what to do next? Should I try filling the cracks, sand and then paint again?

These are the original doors to the house and don't want to have to go and buy any more as the dimensions of new doors do not fit.

Many thanks.