:wall: We have a 1942 house with plaster walls. To repair them, we skimmed with a joint compound. Then to prep for painting with primed with Kiltz latex. The Kiltz is adhering to the wood trim, but is coming off on the walls. What the heck do we do?

I bought a previously lived in home about 6 months ago. I do not believe it has been recently repainted where the issue is occuring. In a corner of the home where drywall meets drywall a crack nearly the length of the seam just "appeared" about the width of penny. There is no corner tape on the wall. Could this be why this happened? Also, If I fix the corner tape issue I am not sure that I have the paint code for what is currently on the wall. I know I can take in a sample to Lowes and they can match it but its kind of difficult since I can't take a wall in. Does anyone have any suggestions for paint issues and/or the cracking? Note: The foundation of the home looks fine, this is a split level home and the cracking took place on the upper portion that actually sticks out from the rest of the house. So maybe some wood has just flexed a little bit or something? I don't know.