Are you using a panel with the glow driver and the starter both being powered from the same source?
I had this problem once. If the battery powering the panel wasn't absolutley topped off it did the same thing you describe.
Finally went with a seperate battery for the starter, and a McDaniel glow plug igniter.
Some engines are really hard to turn over when new. Had an O.S. .25 that I could not turn with the starter until it was broken in, even though the same starter would fire my S.T. .60 right off.
Double check the status of your starter battery.
Finally, how are you getting it started, By hand flipping?

Jetts