Build an appetite

You could focus on incorporating more calorie dense foods that may make things easier to reach your calorie goals. Not sure how you're eating now, but if you tend to aim for low fat or low carb (or other diet type foods like that) - avoid them.

Maybe use more butters (dairy or/and nut butters) and olive oil in your cooking. Include pasta, rice, breads, etc. Maybe throw in a serving or two of ice cream now and again.

Try to figure out if smaller meals might help or if you are doing smaller meals, maybe see if falling back to 3 or 4 larger meals (versus the 4-6 small ones) would help you better.

When I know I need to eat or have signs like dizziness and fatigue but no appetite, I just try to go for whatever sounds the most appealing, even if it isn't what I would usually consider "healthy" or in line with my eating goals. For example, when I'm recovering from being sick, I often crave very salty things and simple carbs so I let myself eat those things. This is something I've learned from caring for a relative who has cancer. Sometimes you just need some calories in your system.

I've found that increasing calories by small increments is more mental than pysiological. It really doesn't take hunger to add, say, a snack of a protein cookie, in the middle of the day between meals. I'd start with that.