RDB Burners not firing, no oil flow?

Hello, I have warmflow boiler working with RDB burners, approx 9 years old. Stopped working a few weeks ago, the oil level had fallen to below 1/4 which I never usually let happen. So I have filled tank and followed all the instructions on this and other sites to bleed the system. The oil is there, a small suck brings it through but not flowing. Any ideas, could it be the pump or pressure in the system? I am not an expert?
Thanks in advance for any help!