Each dragon has a Dragon Power associated with it. Higher level dragons have more Dragon Power. Your total Dragon Power is the total of all the Dragon Power from all your dragons added up.

That said, there are two reasons that your Dragon Power might go down:
  1. If you just merged Level 4 dragons, those dragons (and their Dragon Power) have been replaced by a Nest. Look around for that Nest! Collect the eggs from that nest and hatch them into a Tier 2 dragon, and your Dragon Power should be better than before!
  2. If you played a level where a dragon was summoned into that level with a Dragon Portal, then the dragon is temporarily not in camp anymore, and thus your Dragon Power in camp has gone down. If you find that level, marked by an "In Progress", then go into it and either finish the level or reset it, and the dragon will return to Camp, bringing its Dragon Power with it. (We don't like how this works as it is confusing, and we want to fix it.)