What kind of RCA cable are you using? Some brands have very tight plugs. Sounds like you got it right. It should fit, go ahead and press harder.

Post pics if you can, maybe something is bent. We can help point out a problem if we can see it.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris