I'm Not Sure On What To Do

For starters, if you make your diary public, we can see exactly what you are eating, which will help us help you! But, to answer your question, maybe try to eat more calorie-dense foods? You don't necessarily have to eat a bigger volume of food (again, hard to say because I can't see what you're eating already), but just eat things with more healthy fats and calories. For example, if you normally have toast for breakfast, you could try adding some all natural peanut butter to it, which is higher in calories and fat.

Go heavy on the calorie dense but good for you stuff, nuts and nut butters, seeds, avocado and similar items.... use whole milk instead of skim, go for full-fat options instead of no-fat....

When I'm under, to boost my calories, I often drink drinks with calories to help me out.

However, are you sure about your calorie intake. Studies show that most people grossly underestimate the number of calories they are eating, which could lead to weight gain. Are you measuring everything out, accounting for everything(including drinks) and using nutritional information from the packages of food? No offense, it's just something that is often overlooked.

Maybe you could try eating the same quantity, bulkwise, but choose some higher calorie options. One tablespoon of peanut butter will get you 95 calories, but there's not much too it in quantity and it's very nutritious. Or just a handful of nuts of some kind which are very calorie dense. How about replacing a glass of some beverage you would normally drink with a glass of milk, which is great for your bones. If you're filling up on a big bowl of salad, or something bulky that has very little calories, you can easily get the same in quantity while adding calories by choosing a sandwich instead, or a piece of lean meat and a starchy vegetable like potato or corn.

Good luck!!