Vauxhall Astra 2006 key fob problems?

Hi
I have an Astra 1.4 Club, 06 plate. I am having a problem with remote central locking. The "open" button works perfectly, but the "lock" button only sometimes decides to work! It gets very annoying when i get out of the car and spend ages pressing the button hoping for it to lock!

Can someone please advise me on this. I have already tried changing the battery.

Thanks