Maybe the dip tube. Easy to check. It's a 5/8" tube with a flared end locater right under the cold water intake nipple. Remove the nipple and you can pull the tube out. It should run to the bottom of the tank. If it is broken, You're only getting hot water from the top of the tank. A new on costs about $6 at a plumbing shop. Box stores and hardware stores usually don't stock them because they don't sell that many. If this is the problem, don't worry about the broken piece, you can't get it out and it won't hurt anything.