First, do not exercise as much as you currently do. Then, eat in larger portions. To gain a few pounds easily get a high calorie containing such as Nutella. Basically just eat in all the candy and junk food you can but still eat the same healthy regimen. In addition to this maybe get a glucose juice twice a day like Glucerna or Ensure. Try buying some energy bars with protein. Drink whole milk every day if you do not do it already. Lastly, multivitamins taken as directed on box will help a lot. In a month if you do this, you can gain at least 20 pounds easily. If not, talk to your doctor as you might have type 1 diabetes, or other insulin problem. Finally, look at foods containing much fat, carbohydrates, and calories plus sugar.