Oh golly!...

Um... well... I'm not actually very sure to be honest...
I know that carbs & sugar and the essence of "weight"
Fat stores unused sugars. That is what makes you fat.

Eating fats actually makes you skinny... they burn slower are a better source of 'energy' because of it... and good fats bind with bad fats and remove them from your body... so...

do you have a sugar 'problem'? like hypoglycemia
This would make it very hard for you to gain weight as the sugar stays in your blood stream longer than most people.
If you don't know... it would be good to do some research on what it is and see if you have those symptoms. If you do have it you'll need to balance your chemicals and then your weight will be able to regulate it self.
I take a product called Gluco Balance by Futureplex...

also if so, you may want to eat more fat and protein early in the morning...

Best. Let us know if you find the answer for you...

Hi .......... I would suggest more nuts (especially almonds ) pumpkin seeds, sesame seeds, fresh coconuts, organic cold pressed coconut oils & olive oils . You should really soak any nuts & seeds first.

If you are eating properly, and being highly active, it's likely your body is just naturally supposed to be that thin. If you want to build muscle, do more strength training as opposed to cardio. I'd encourage you to use natural body weight as resistance, and avoid muscle-specific machines as much as possible. Natural motion is the best for both safety and health. Your body will naturally take care of the rest if given proper nutrients.

When doing lots of exercise, you need energy-rich fruits before and immediately after to keep your sugar levels stable. Pick out a variety of delicious fruits and enjoy. :) After your exercise, your body needs amino acids and proteins to build muscle and repair damage to any tissue. Nuts, seeds, a variety of vegetables, etc.

A general rule about diet- your body needs more carbohydrate-rich foods in the morning, and less and less as the day progresses on, while it needs very little protein in the morning, and more as the day goes on. So:
Breakfast- high carbs, low protein- fruits, oatmeal, etc (your body needs energy to get moving, and it just had time to repair during sleep)
Lunch- balanced nutrients, a little fruit, softer vegetables like corn or squash, grains, etc
Dinner- higher fat and protein, low carbs. Lots of nuts, seeds, a variety of vegetables, etc. (your body needs proteins and amino acids to repair itself while you sleep.)

Most people make the mistake of eating tons of rich, sugary desserts right before they go to bed- the absolute worst possible time to put carbohydrate-rich foods into your body. You need lots of quick energy to... sleep? I see people all over do this, then wonder why they get fat, lol. A little off-topic, but still important.

The best book on diet I've read was Body by God, by Dr. Ben Lerner. He's a nutritionist that specializes in Olympic athletes. I'd highly recommend it.