Partial blockage in pilot assembly will cause it, thermocouple not getting hot enough. Probably from lack of service or too long between service periods.
Cracked or split thermocouple will cause same problem.
If its electronic and has flame rectification, the flame rod being dirty, and bad earth are places to start looking.
Be wary the masport mg3000 series of heaters, they are famous for delayed ignition, and exploding glass.

Yes. Glen Dimplex.