These problems are caused by under proving a loaf before baking. Make sure that the dough has swelled to 85β90 per cent of its original size before baking
otherwise the crust can fly upwards in a balloon effect, separating from the dough, and leaving a gaping hole between the crust and the loaf. It is also the cause of a seam of what looks like uncooked dough in the bottom 5mm (ΒΌ in) of a loaf and will leave you with dense-textured bread.