Truma Heating Trumatic S3000P doesn’t want to…

Hello forum, after we have always been camping with our Ami Van in the last years, we have now bought a small used WW. It also works so far to the Truma, which makes some teasing. I got it to run 2 weeks ago while testing after a longer try and was actually of the opinion to make this again. I tried it again yesterday but it was nothing to do. It is a Caravelair WW and the heater is a Trumatic S3000P, which was produced only for the French and English market until 2000. It has a piezo ignition by hand , which also works , but it is unfortunately no ignition process of the gas to achieve , or is no gas there to ignite . The gas taps are on , the stove and the cooli work fine . How can one bring such a heater back to front man , disassemble , clean and what better should be left . By the way , gas testing passed the WW 3 weeks ago . Thanks … !