If your oven is too cool for the type of dough you’re baking and/or you don’t leave it in long enough, it won’t bake through properly. Your loaf might not only be dense, it could also be raw in the centre. If on the other hand your oven is too hot for that type of dough, it could bake or even burn (a particular risk with enriched doughs) on the outside before it’s cooked through. Some professional bakers use dough thermometers to be sure that loaves are fully baked at the centre.