I suspect it is a faulty vacuum pump which controls the central locking. You can do this to narrow down the problem.

1) Does the same happen when you try to lock/unlock from the pass door using key or push/pull the plunger?
2) Does the same happen when you try to lock/unlock from the trunk?

If it happens with (1) and (2) then it is probably the pump. It it does not then it may be the driver's vacuum pod wiring not latching.

Good luck.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.