Hi there,
You may have the same problem I encountered a few times in my C200 1996. The
central lock simply blocks itself from time to time nad losses connection
with remote control - with no obvious cause. Car manual covers that question
and tells, that in such situation you have to manually (with key) open front
passenger door, somehow sit on drivers seat, turn key to ignite all controls
on front panel for a few seconds (but do not ignite engine), turn key off,
press "open" button on remote control. Red and green lights will blink -and
thats it, your keyless entry now working!
That was, what helped me - maybe it'll help you as well....

Andrzej