The easiest one is to simply force down more food, i have the same problem, i am very rarely hungry, but my metabolism is high.
So i used to end up eating too little, and then my body would attack itself by eating up mucles and the little fat i have for energy.
So i simply force down food!
Not hungry? Eat!
Don't know if you have the same aggressive metabolism that i do, but pushing down calories seem to work for me atleast, gained 8kg (17.6lbs) the last 2 months.
And the more food you force down, spread across the day, the more used to it your body gets, so eventualy you will get hungry several times a day cause your body gets used to always having something to digest.