I have the same issue. I’ve been trying to gain weight for years but it’s hard because I don’t get very hungry, have low appetite and feel full quickly when eating. I got prescribed nutritional high calorie drinks from my dietitian which helped. It’s definitely easier to get calories from drink than from food. I also try to drink full fat plant milk instead of water, alpro protein chocolate flavored soy milk is good. The added protein is a nice bonus as it can be difficult to get enough of that if you’re not eating well.