Very basic new to plumbing question/problem

[NewDIYinOH]

I replaced the PVC plumbing under my bathroom sink when I took everything apart to get something out of the pipes. The PVC connects to a not PVC pipe at the wall. I have a 1.5" slip washer and slip nut in place but I cannot get a good seal so I have a leak at that point. It seems like the slip nut is not tightening properly and I cannot figure out how to get it to screw on evenly. I tried without the pipe and to get it threaded at all I have to exert some pressure towards the wall but I can't seem to do this with the pipe in place. There was no leak prior to my taking everything apart. The pipes did not use a reducing washer at this connection as far as I can tell.

I realize this is such a basic issue but it is keeping me from completing my project. I can't figure out if there is something else I need or if this is simply inexperience.