As soon as the fan starts blowing, using a small hammer moderately tap the control valve . They sometimes stick .
Also, i'm assuming the TT is level.
I had an RV furnace that would not light if I was parked nose down. As soon as soon as I leveled it, it fired up .
I never heard of that happening before and my rv mechanic looked at me like I had two heads when I told him my fix. But it happened all the time I was parked nose down, furnace wouldn't light while all other gas appliances worked fine.