White flour won't rise

My 100% wholemeal wheat sourdough rises well and makes a fine tin loaf. I use a wholemeal rye starter, 100% hydration, refreshing it overnight and incorporating it in the morning.  I've tried to make a white flour free-form loaf but am always disappointed as the fermentation seems to lose vigour, the dough becomes increasingly flabby. I'm puzzled as white flour is usually much easier to work with in a yeasted dough. A yeasted dough in a banneton rises well.  Why not my sourdough? 

Any ideas?