How can I fix my key fob for our vauxhall astra?

Vauxhall Astra SXI 05 Reg. Very often when we remove the key from ignition and open the door , the 'door open' light starts bleeping but this should only bleep when the key has been left in ignition. Then when we get out of the car and close the door the central locking will not work as the car still thinks the key has been left in the ignition. We have changed the battery in the fob but this has not solved the problem.