Bez44

I have a very mature apple tree with 3 main trunks / boughs. It is usually regularly pruned but was not done last year. All the branches on 2 of the main boughs have leafed and blossomed as usual, but those on the third have just begun to blossom and then stopped, as if the tree is low on energy. Should I trim the branches on this part back a third to try and conserve energy and encourage growth on the rest of the poorly part, or is it a lost cause?