Vauxhall Astra Remote Locking?

I have a Vauxhall astra that is coming up to 10 years old. A few months ago the remote central locking stopped working. The light on the key still comes up but still nothing happens. I re-synchronise the key in the ignition the car then locked and unlocked its self. I pressed the lock button on the key and then it started working again. A few months since It suddenly stopped working yet agai. I did the same thing with key. This time it did not fix the problem despite the light on the key still works. What should I do?