Ours pairs up by hitting a small button on the box under the seat in the caravan. We were told to place the handset close to the box and hold the button for a few seconds. Ours is a good few years old though.
There is always has developed a possibility the handset has developed a fault, ours did and we had to have a new one.
If you ring them at Truma they will talk you through a few checks, we found them very helpful.