Blennies can be picky eaters, fish like most will get bored of the same food. I recommend to continue using the food you are, live and flake foods are good for fish with the difference in nutrients it provides. But in the case of your fish having difficulty eating crush the flakes between your fingers first as you sprinkle them into the tank. If the problem persists i recommend changing to treat your fish for a possible case of constapation which can be a problem in fish who have eaten too much or too little.
I would leave it a few weeks before considering any other actions, just feed the fish as normal.