problem gaining

ive been the same weight for almost 1 1/2 years now....im 16 years old, male, 5'11". I am weight training and i eat a good amount of food every day....it just feels like ive been maintaining for too long now. It it normal for someone my age to stay at this weight for a while? or should i be gaining anymore.  

If you really want to gain, but aren't gaining, then you aren't eating enough.

Some people are ectomorphs and require unbelievable amounts of food to gain weight.

it's normal since your metabolism is like a hamsters! stick to a high calorie diet with proteins and carbs and you will start to grow eventually.

ok...ive been 140 lbs by the way...

Yup, eat.  At 5'11" and 140 lbs, you probably are an ectomorph, and a very high metabolic rate often comes with that.  I sure hope you enjoy food :-)  Boost your calorie intake 10% a week until the scale starts to rise.  The most effective calorie range for you to grow muscle without too much excess fat is then probably between that and about 500 calories/day above that.  Nobody can tell you for sure in advance, but it's straightforward for you to find out by experiment.

eating sounds good....i guess i could be an ectomorph but i dont look toooo skinny by any means....kinda inbetween an ecto and meso...but leaned more towards ecto at the moment.

Well, body type really doesn't matter here:  you just need to eat more regardless :-)