Fats are an easy way to increase calories without feeling like you are stuffing yourself. Nuts and nut butters are a great source of healthy fat.
I'm having the exact same problem! For the past few days I've been not getting enough calories, although I've been eating enough food to keep me full throughout the day. I munch on veggies or at least the healthiest foods I can find, and at the end of the day I realize I've not nearly reached enough calories.
I like amethystgirl's suggestion about the nuts and nut butters; it's something that I've been trying to do! Instead of reaching for even more vegetables today, I made myself a peanut butter sandwich and made sure I had some healthy (but full of calories) nuts.
Good luck with increasing your calories! :) remember, it's not always about how much you eat, but more so about WHAT your eating. Don't eat less amounts because you think it may be healthier, just eat higher amounts of foods that are still nutritious.
peanuts are legumes, not nuts..!
be careful with peanut butter as most of them off the shelf are full of high fructose corn syrup, and you really don't need that at all... look for all natural peanut butter without any sugar! and try some almond butter... almond butter is deliciousness,