banneton problem

I recently made a wholemeal spelt/rye/wheat mix at 65% hydration which seemed to go well including shaping and I put four loaves into 4 small bannetons which I prepared using coarse semolina rather than flour.  They proved ok but when I had turned them out onto hot baking sheets to slash them I found that the loaves had set a skin which meant that the slashes didn't widen properly and the loaves burst  somewhat in the oven.   Is it likely that the problem was that the semolina didn't seal the dough into the ridges of the banneton?  I'm not sure why I used the semolina which i normally only use to sprinkle on the sheets to prevent sticking!