Sime Halstead Trio 30/90 - Main Burners are taking too long to fire up?

Everything seems fine; Pump is on ; Fan starts; Pilot light come on, Flue is clear but burners are taking sometimes taking 5-15 minutes to fire up.
The burners do eventually start.

Any Ideas what could be causing this?? Is there a problem with the Pilot Light injector or something causing else that could be causing this annoying delay. It happens when HW tap is turned on, and also when CH is switched on.