So recently I did the alternator on a 1999 S80 T6 and it ran till warm and my fuel line popped off.

I put it back and pushed harder and heard a click, then my fuel pressure regulator which also just pushes in came out.

I put it in and heard it click too, but it just came back out today.. Why is this?
It has a brand new O-Ring from volvo the proper one.