Car fob not working properly?

I have a Citroen c5 1.6 diesel.2009model. The fob won't unlock the door, when clicking it nothing happens. You have to use the key, unlock drivers door. When you start engine mirrors etc start up. You can use fob to lock the car and open it instantly using fob. But within a minute fob again is useless and won't do nothing. It's a new battery in fob, so what is going on. Mechanic plugged it into machine and no fault is showing up. Please someone help