It's funny how these heaters of ours all seem to have their temperamental aspects
Mine is a Propex and I can guarantee that every time I initially turn it on after a longish period of inactivity it won't work! However a quick hard blow up the heater burners air inlet pipe with air from a high pressure hose, or a suck with the vacuum from the heaters exhaust hose always clears the problem.
When away from home on a remote CL or 'wilding', the small but powerful Black & Decker vacuum cleaner I carry in 'Millie' has a pipe which is a perfect fit for the heaters exhaust pipe and so a quick 15 second suck soon has the heater working perfectly!
Phil