Bear in mind flash is a far more contrast source of light than natural sunlight (In most cases) which is where a problem may lie. However it looks by your description that your flash source may be a little optimistic about the level of light output. Was your flash controlled by the camera, by the flashgun or by using a meter? Any of these could be giving a wrong estimation of the requirements for the film. You could try rating the Pan F Film at 25 ISO and see what the results are like then.