It'd be worth checking teeth & do a worm count if you haven't already or use double dose Pyrnatel which will do tapeworm too as it's spring.

I managed to get a TB mare with a history of laminitis from ribs easily visible to showing weight in 10 weeks. The horse had been out on loan & came to me in a hurry the day after a routine visit!

I fed a good quality feed balancer @ 1g/kg body weight - mare should have weighed 500kg so fed 500g balancer together with a yeast based probiotic together with small amounts of chaff and unmolassed sugarbeet for her bucket feeds, 2x day. She also had tubtrug with 3-4kg high energy chaff (mix of mollichaff showshine, alfaoil & dried grass) at night... looks loads but built up gradually and she soon learned to pick at it through the night. Ad lib hay (4-5slices) & 8 hours grazing. It was spring so nature leant a helping hand!

In my experience the less carbohydrate/cereal you feed the better doers they become.  Many of the conditioning feeds on the market are based on cooked ceral meal or extruded cereals e.g. barley... which contain the calories but in %age terms most of the horse's gut is large intestine which digests fibre. Cerals need to be digested in the small intestine if they're to be absorbed but horses have very short small intestine, relatively speaking. The probiotic supports the gut bacteria working in the hind gut so helps horse get more goodness out of the feed it's eating. My 25y/o TB hasn't had cereal other than the tiny bit in his balancer for 10 years & he thrives on 2 tiny feeds plus adlib haylage & still is working. 15 years ago he was being fed mix, chaff, sugarbeet & oil 4x daily & still strugglied to maintain weight.

If teeth are an issue that the vet/dentist can't solve then it's still possible but more expensive. Find high fibre conditioniing cubes & add water to saok them to a mash, which doesn't need to be chewed. When the weight is on you can then change to high fibre cubes. If you ask feed companies for samples of balancers you can check which of those will soak to a mash. Alfabeet & speedybeet & kwik beet all soak to mask with v small particles so won't need too much chewing. Some brands are harder than others - the harder the nut/cube the hotter the water needs to be in order to soak it. I also suggest you soak in a tall/thin container because when the water spreads out further it cools down quicker.

make sure she's wormed, and do a worm count to see which worms she may have.

Check her kidneys and liver too - with the use of a vet.

hope this helps. At her age, worming is often overlooked (or forgotten) and kidney and liver problems can start occuring.