How can i fix the door locks on my Mercedes C180 Kompressor?

The front door locks on my Mercedes C180 Kompressor estate (52 plate 2003 model) have an intermittant fault. When both locking or unlocking using the remote fob the front door locks bounce up and down and either don't lock or don't unlock depending whether I'm locking or unlocking. Sometimes the locks work fine, others can take 5 or more attempts. Has anyone come across this problem and what is the fix? There are no faults coming up when checked on diagnostics.