Not sure about that particular shutter, but some Ilex shutters that would be used on a 12" Comm. Ektar have an adjustable delay for electronic flash or for bulbs of various types. If the sync setting says "M, X, Off" it should be on "X". If you've got a setting that says "Blue 20, White 5, Red 0" it should be on Red, and there may be a separate lever to cock for the flash sync. It sounds like your flash is synched properly, and it is probably the case that you just aren't exposing sufficiently for paper, as the previous poster says (figure exposure at ISO 2-6, and you could be getting high-speed reciprocity effects with flash, so maybe you need even more).