Why does my central locking only work some of the time?

I have a W reg laguna and have had the same problem for 18 months i have been to renault for diagnosis they said i need new keys i waited 3 weeks and the keys didn't solve the problem so it must be something to do with the receivers in the unit with the lights in near the mirror ( they sometimes get damp in there but mine is dry as a bone ). i sympathise with you its a pain in the A hole.