I gather the alfalfa is the hay she gets? Does she have any grazing/grass hay? How much alfalfa, grass, hay? How is it fed & how often is her hardfeed fed(how many meals daily)? Does she get any nutritional supps?
Horses are built for eating small amounts of poor grade(compared to cattle fattening grasses & high starch grains) high fibre roughage near constantly & they need a minimum of about 2-2.5% bodyweight daily in roughage, more if you want to put weight on. So I'd first consider adding more (grass)hay to her diet.
They don't deal well with big &/or infrequent 'meals' particularly if starchy/sugary - their system doesn't deal well with high starch/sugar diets. Therefore I'd feed any hard feed in at least a couple of feeds per day, more if it's a large quantity/starchy feed. Speaking of starch & sugar, I'd also ditch the TC sweets in favour of something healthier - not grain rich or smothered in molasses.
Alfalfa is a good, energy & nutrient rich feed for horses generally. But it is very high in protein, calcium & such, so needs to be fed *as part* of a balanced diet. Nutritional imbalance/deficiency can be a reason for a horse's 'failure to thrive' even when calories are more than adequate. FeedXL Horse Nutrition: The D.I.Y. equine diet planner
is my favourite equine nutritional source.
Considering ulcers, stress & management are also important factors - eg if she's in a chronically stressful situation(physical or mental) this can effect her health including weight. If she's cooped up in a stable too much or otherwise restricted from free exercise, this can affect digestion too.