I guess it had to happen, but it's still rather surprising.  It's an almost all ww bread using my very healthy and active white starter and going by a Bernard Clayton recipe which has added yeast.  It took forever to rise and then not very much.  The dough was heavy and not easy to work with at all; it's in the oven now, in two small pans, no oven spring, and I'm not confident of the outcome.  Curious, but I think that's the nature of sourdough, isn't it?  I can't be high on bread all the time, some bricks will be made, and I just go on from there.  Still, I wish I knew why.  The only thing I did differently from the recipe was to add a bit of sugar.