Flash underexposure issues

I was on a trip today [EDIT: well, yesterday by now] to the Transport Museum here in Glasgow (not as an exercise in photographic skill, but really just to snapshot the place since it's shutting down in a couple of weeks)

My issue is that for some unknown reason, the combination of my K20D and Metz 48 produced some very underexposed images (by 2 or 3 stops in some cases). Now this wasn't a case of me asking the flash to do something unreasonable: if I used the on-flash exposure compensation, or manual mode, there was enough power to produce a correct image at the same aperture/shutter speed. The other point is that it wasn't a consistent thing.

Whether because of the slightly odd lighting in the place, sometimes it needed +3 EV and sometimes that would produce a very overexposed image almost blown out because of the reflectiveness of a lot of the stuff - this difference could be taking shots in different directions standing in the same spot. Is this something to do with my technique, the equipment, or just a freak occurrence in a weirdly lit museum?

You already said it reflections, it try not to overexpose those so that would lead to an underexpose photo. Besides that most Pentax underexpose the flash around the 0.7 to 1 EV standard.

Stefan's got it IMO - a bright reflection will cause the flash to shut down earlier than it would otherwise.

Was any of your subject material in glass cases per chance?

Some of it was as a matter of fact - and most of the rest of it was made of metal