My 8month old Ibizan Hound won't gain weight and people at ring craft have commented on how thin she looks ☹. She isn't interested in food and will happily go days without eating. I changed her food and she wolfed it down day 1 but now day 2 didn't eat any thing. I feel I am being made to feel like I starve her, when this is not the case, I have tried little and often meals as I feel she gets too full.
Many years ago, I had a Cavalier who would never eat a sufficient amount to look finished enough to show. I used to top her up with ginger biscuits, and anything else she showed interest in, but she remained healthy but underweight. Around middle age I had her spayed, (she turned out to only have one ovary, as did her only daughter, but I don't think this was connected), and she went on to develop a normal appetite and bodyweight. Always very fit, I lost her with a brain tumour aged 12.
I had a bitch like this, and she only improved after maturity and a litter. The trick is not to be tempted to give a bigger meal when they choose to eat, as then they are not hungry and refuse next meal until you offer something novel. Giving several meals of a high quality nutrient dense easily digested food. Do not give treats between meals, offer the food for 15 minutes, and pick up until next meal time. Do not make a big deal of meals, leave the room so she doesn't pick up on your stress. Has she had a season yet? Mine was worse during and just after seasons. She had a litter at 2, ate really well while rearing pups, but back to picking on next season. I stopped fussing, did as above and she gradually improved by three years or so.
Have you got any pictures of her? Beezers are skinny little creatures anyway as I am sure you know. Maybe take her to the vets to get a blood panel done so you have more of an idea what may be causing it.