I lift weights and eat plenty but I still have trouble gaining muscle and I often lose much of the weight over the summer. Any tips on how to build and maintain bulk?

How old are you? If you're 17+ try buying some supplements ! Different types of supplements will help you out when working out in different ways. To keep that muscle you gain just keep working out and get the right nutrition, its simple ! Make sure your getting enough Protein and BCAA's ( maybe you want to buy a type of protein powder ). Protein powders can taste pretty good too ! I recommend 100% whey protein after you workout because the body digests this the most quick.