Too much salt? Or, I see in one comment you mention four hours of proving - are you over-proving the dough in too warm a place? The loaf might be collapsing in on itself as it bakes. Depending on the flour used, I find one-two hours is sufficient for the first prove, at ambient temperature; if it needs a second prove after punching down, around 45-60 mins will do. Some doughs can be proved in the fridge overnight, but there tends to be specific recipes for those.

Is your yeast fresh ? And flour?