Humm ... could be a couple of things. But, here is my list of thoughts:
Like Claire said - the food must be dry. Pat it dry, dredge in flour and shake off the excess, then in your egg/milk wash. and then in your final coating (flour, Panko, bread/cracker crumbs, corn flakes, whatever). Dredging in flour first insures a dry surface for everything else to stick to. Some people suggest letting your food sit on a wire rack for 20-30 minutes after this to allow the coating to dry out and set before frying. I don't always do this but it can make a difference.
The best temperatures for frying are from 350°F to 375°F. You need to get the temp up high enough so that it doesn't drop below 350° when you add food. So, if you mean by lowering the temp when frying some things that the temp will fall below this point - that is one of your problems.
You do NOT want to put a lid on something you are frying if you want it to be crisp. This causes the breading on top to steam and become wet - this can lead to the problem you describe. If spattering is a problem, get a spatter guard
to use instead of a lid.
Don't overcrowd your pan. Not only will it cause the temp to drop too much, it also causes the food to steam - there has to be space between the pieces of food for the steam to escape.