hello

recently got a genesis 2nd hand and it seems as if the roto start will not turn the engine over, when i remove the glow plug it works fine and tunr the engine over, but when i put it back in, it struggles to turn the engine offer. I had a look at the one way bearing and cant see that anything is making it slip, The rotostart is fully charged so there must be enough power to turn it over.

anybody with a clue please reply.
Thanks in advance