My car they lock fine.... but I suspect you have a vaccum problem like Mbenz suggested.
Look for the vaccuum lines under the seats and make sure they are not broken or disconnected.
Everything else is ok though right?
Door locks and all function properly?
IF you are having door lock probs too, then there is probably a vaccuum leak.
Thats my opinion on it.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.