I`m presuming its an electric flush model, Christine.
If so, the most common problem is the pump, which you can access by removing the toilet roll holder recess, then you can get to the pump. These are available from your caravan dealer, just make sure you waterproof the connections when replacing it.
The power to the unit will also go through a fuse (ours used the same circuit as the water pump) which is worth checking, and if all this fails you are down to two microswitches, one at the base of the `plunger` you press to activate the flush, the other acts as an interlock to prevent the flush being activated if the holding tank is out. Unfortunately, you have to remove the entire unit to get to these two.
If you go to the Thetford website you used to be able to download `exploded` diagrams which showed you what was where.
Best of luck.