One obstacle to a back rank mate, as we have seen, is that your opponent may have pieces of his own guarding the square you want to use. Here is a different problem to consider: he may be able to interpose a piece on the back rank, sticking it between your attacker and his king. Sometimes such interpositions will derail your plans; sometimes they won’t. You have to play through them in your mind’s eye to see whether they can be overcome.
In this first diagram, White can't mate. Since Black’s king stuck in the corner White naturally considers 1. Rd8+. The d8 square is unguarded, but there is another problem: Black can throw a rook onto e8 and block the White rook’s path. Since the one Black rook would have protection from the other, White would not be able to get past it; 2. RxR+, RxR and White merely has traded rooks when he already was losing.