I have the same problem on my 1997 Hyundai Accent Coupe. I installed the central locking kit myself so I know that the connections are all good and both the remotes work. It worked fine for the last six months but a couple of days ago it just stopped working. The funny thing is it will lock via remote if I manually unlock both doors, but it won't unlock again.
Anyway, back to answering your question. I think maybe the solenoid motors in the doors may be frozen/not functioning properly. The cold weather the only thing I can think of that has changed recently in my circumstances.
What I'm gonna do is pop both the door cards off and get to the motors and see if there is an obvious visible problem. If there isn't one (and I'm sure there won't be) I will spray some WD40 onto the motors which should keep the damp out. If that doesn't work I will be very p*****d off!!!!! And I'll have to back to the drawing board. Anyone else got any ideas?
Rob C
November 2005