Gas fire installation

It could be the THERMO couple or gas valve obviously the engineer checked to make sure nothing had come loose after fitting and too be honest any problems during the guarantee period are the manufactures problem so the engineer made the right decision to pass it on to them

As Chris Holloway says, the thermocouple or gas valve could now be playing up, also, some gas fires require a restrictor plate to be fitted to stop excessive flue pull. Excessive pull could pull a modern oxypilot flame away from the thermocouple tip as, even in normal usage, it is designed to only just glancingly touch the thermocouple tip and cut the fire out if there is any issue. I fitted a gas fire yesterday and because it was a proper brick chimney it stated that the restrictor plate must be fitted. If it was a precast chimney then it (restrictor) was not to be fitted.

could be as above, loose thermocouple connection or could be caused by the fitter not purging the pipes prior to running the gas through the fire, what can happen is bits of flux and grit can be introduced when soldering the pipes up and this can get into the very fine injector at the oxypilot causing a partial blockage which then effects the pilot flame which isnt a strong enough jet to then heat the thermocouple up. The latter is the cause of a lot of call outs for manufacturers engineers just after a fire has been installed

It could be that the gas pipework had some dust or debris in it so the debris has travelled into the pilot and semi blocked the pilot so that the “small” pilot flame isn’t playing correctly on the thermocouple.