I had the hot water side on my shower break. I got the new parts the are Delta and now it won't go back together it is like it won't go in far enough to lock in. Any idea what is going on?

So it sounds like you bought a new cartridge, right? Does the new look exactly like the old including model number and O rings. You usually need to apply plumbers grease inside the valve body before insert the cartridge.

Lube your cup gasgets and make sure they are started in the holes before inserting your cartridge

If you have stem that uses a compression washer, that is a stem with a washer on the end of it, install with the stem in the open position. That is, with the stem turned just like you turn the water on.