Ensure you have her on a consistant worming schedule and ensure any feed you are giving her has at least a 14 percent protein content. My mare Breeze had the same problem and is now filling out after trial and error(and feed education on my part)have had her since she was 6 months)...one problem was making sure I wormed her every two months (and she was horrible about taking it, but has improved with consistancy)and they need a higher protein usually to keep weight on. I switched all my horses to Strategy two years ago because of the higher protein yet still not alot of sugars. You can also buy weight/yearling supplements to put in their food also. I only feed once a day (work schedule) and she gets a scoop and a half (if they are not used to that much gradually increase it) and they are out on pasture all day with access to round bale (coastal here in texas) all day. Just as a comparison I have my 9 year old walker on just a 1/2 a scoop of feed because he just looks at food and gains....poor boy. Perfect schedule would be to break it up to 2 or three times a day but doesnt always work with some schedules. When she was a year she was on a scoop in the morning and one in the evening and probably would have put more weight on but back then but I was just feeding 10 percent feed...sometimes it is just hard to keep weight on the youngsters because they have alot of growing to do. Just when I would get weight on she would shoot up in height again and would start all over....hope this helps?