TV to Home theatre

I have a Kenwood stereo of model number UD-505. I have attached two main speakers to the unit, as instructed in the owner's manual. I have then attached a centre speaker to the surround speakers port The speaker is hooked up to my TV through an RCA connection. I've been having a problem where whichever mode I choose, Dolby Surround, Pro Logic or 3-Speaker, I can't get the speakers to all work together. The large speakers work and the small centre speaker doesn't or the other way around. This speaker system in about 10 years old, so I haven't had any luck finding help anywhere else. The three speakers do work together on FM channels, so what input would you suggest?
Fingers crossed and thanks in advance.