I usually see a little foam on top of my (stirred) starters, but never a true krausen, as it doesn't have any yeast gunk in it. I think Robert is right about the stirring, which would prevent, or at least minimize, the accumulation of a true krausen in a stirred starter.
The only time I've ever had a true krausen on a stirred starter was when, overnight and unbeknownst to me, the starter stopped stirring, as the stir bar moved off the center of the flask bottom and was pinned against the side. So a krausen formed, and of course as soon as I moved the flask it gushed everywhere as the CO2 suddenly broke out of solution. Perhaps this happened that one time with you?