I can only tell you what I've been experiencing with my own "problem child".
We brought her home 3 years ago when she was 11. I'm her fourth owner. Her previous owner had her for just under 2 years, I believe if I remember the registration information correctly. She was aloof but seemed to settle in that first spring/summer/fall. When she first came, we had one other filly (11 months old). We added two more weanlings (August and again in October). She was the boss mare from the moment she came here.
That winter it was obvious she wasn't holding her weight well. She got cold when the others didn't seemed bothered at all. We were so green though that some of this wasn't as obvious to us and I see it now with retrospective eyes. When spring came and their hair started shedding, I was shocked with how thin she was. Pointy hips and her back had a definitive ridge. Her hair coat was poor.
Over the past two year, I've tried a bunch of different approaches, with some success. Still her hair coat has never been great (especially when compared to the others). She doesn't maintain weight well. She was getting 2-3x the feed, 16% sweet feed with calorie/fat supplements. She should have been HUGE. I also tried U-guard and felt it had some benefit. We also switched to an alfalfa mixed hay for all the horses early on. Our second year we didn't have much of a pasture so I gave them hay, but she won't fight for her food, despite being boss mare in ALL other aspects. She hates being stalled, so separating them is difficult, but we do it.
We came out of this winter and I was feeling somewhat optimistic. Then last month, she practically quit eating ALL her grain/sweet feed. Even on the pasture, she would eat and then stop, hanging her head. Or laying down (not flat out though). She looked like she was utterly miserable. I had the vet to a baseline panel of blood work last month and the only thing that came back as potentially "off" was a protein level which was really low and when you look at what she was eating before the bloodwork was done, it didn't make sense. But the vet said it *could* be an indicator of ulcers.
After watching her decline for 2 weeks, I contacted the vet again. We decided to start a course of Gastro Guard. We chose this brand because it has increments of 250lbs on the tube which makes it easier to dose her (she weighs roughly 195lbs...I have all my horses weighed every summer).
In addition, I came across information I had previously forgotten. If ulcers are suspected, sweet feed is the WORST thing to feed a horse. In fact, my research has repeated shown that ANY grain should be avoided. Pasture and hay, with supplements if necessary are really the best for a horse with ulcers. And constant access to forage is key to keeping stomach acids down as well.
We're starting our 3rd week of the Gastro Guard. I stopped trying to feed her the sweet feed about 2 weeks ago...she was barely eating it anyway. I'm giving her a daily ration balancer supplement only. I gradually increased her pasture time so that she's on the pasture about 12 hours a day. I'm actively trying to find a small structure that we can afford to purchase for the pasture so she can be down there 24/7. I cannot let my others out like that though, thus the need for another shelter.
My vet's recommendation is to do a 28 day treatment with the Gastro Guard. We will reassess where we think she is at that point. IF we think she's still not markedly better, we're going to test for Cushings and thyroid levels.
The Gastro Guard will cost me $234 by the time we're done this first round. Not cheap, but better than the $39 per *day* my vet is spending on her own horse who needs a whole tube a day for 28 days, then goes to 1/2 a tube for 28 days and then another few weeks of every other day dosing. She's doing this because he was scoped and confirmed with an ulcer. Getting my horse scoped is not something we can easily do...finding someone with a scope that will work on her is an issue.
I'd talk to your vet. If you think ulcers are a possible issue, I'd personally go straight to treating it with Gastro Guard or Ulcer Guard (omeprazole). I've spent 2.5 years trying to puzzle this out and ended up here anyway. I'm cautiously optimistic that the meds and the changes I've made are helping. I'm starting to wish I had done this sooner.