One of the fairly regular problems I encountered with Trumastores (before they were called Ultrastorses) was the fact the burner seemed to attract insects who would make their nests in it. The same applied to Carver Cascade's also.
If they construct a nest or create any other sort of blockage, in the burners venturi or the the burner plenum, it can either prevent ignition or stop the flame from spreading properly and affect flame detection.
In the case of the Truma boilers, the burner is almost a complete circle, and the fame would burn around about 150 degrees of the circle. the missing portion is where the gas and air enters in the venturi tube. The flame would be ignited on one side of the venturi, but the flame detection was on the opposite side of the venturi, so the flame had to ignite all the way round the burner before it could be detected.
One of the characteristics that often was a symptom of this, other than poor ignition or flame detection, often the appliance would start to produce soot.
If you have any evidence of sooting, its a clear sign the appliance is need of a service. For your safety it best to leave all gas appliance service work to a professional.