Why Can't I Loose Weight?

I think I eat pretty healthy and work out a few times a week on my elliptical (also do about 5 mins of abs and 10 mins for my arms on the total gym) and have been doing this for several months and have not been able to budge my weight! I am 23, 5' 7" and between 140 and 145 lbs. I don't need to loose a significant amount of weight, just 10-15 lbs. so that my clothes aren't so snug and I want some tone/definition, especially in my abs and thighs.