Sigma M system key problem?

I have a Sigma M system on a VW Golf. The remote key seems to have stopped functioning. Couldn't unlock car so used the lock key; alarm went off, & the remote key doesn't stop the alarm either. I've changed the batteries, and the light on the remote illuminates when the button is pressed (so electrics working), but it doesn't seem to be transmitting to the car. What's the solution?