sibrear

I just bought car so have no history. 72,000 on the clock. I have replaced key fob sender and attempted to activate lock system at all three points (trunk, right door, rear view mirror) with no luck. Central locking works with key and judging by blinking light at dash, alarm works. So, what could be wrong? I suspect something central as I doubt that all three receivers would have failed. Any comments would be greatly appreciated