From Google
There could be several reasons why your piezo igniter is not working. Here are some common reasons and solutions:

Battery: If your piezo igniter is battery-powered, the battery may be dead or low. Try replacing the battery with a new one.

Wiring: Check the wiring of the piezo igniter to ensure that it is properly connected and not damaged. If the wiring is damaged, you may need to replace it.

Obstruction: The spark produced by the piezo igniter may be obstructed by dirt, grease, or other debris. Try cleaning the spark tip with a soft-bristled brush or compressed air.

Cracked ceramic housing: If you are clicking the piezo and you are not seeing a spark, this is usually the problem1. The fix is easy so dont buy a new ignition just yet If none of these solutions work, you may need to replace the entire piezo igniter.

I hope this helps!

I have just finished fixing my Thetford fridges piezo system. I have found that often it is as simple as a soot build up on the piezo wand that sits just above the gas burner and it is the contact point that the spark jumps from. I have found that if you give it a few gentle flicks with a stiff wire and knock the fine soot off it will usually fix it. Failling that, a very careful lite brush with a needle file around the area of the contact above the burner should do the job. Also the spark gap should be about 4mm so check that. You can also play a bit with that. I usually have to do this before each trip and it is normally fine for the trip until next time.