I'm worried about my four year old beagle (again!!!!)

She's thin. I took her to the vet last week for her annual shots and raised my concerns, asking him how I can get her to gain a couple of extra kilos. He said she was optimum weight, and things were fine, so no advice was given. But he did say any thinner and they'd look into it

She devours food, has been wormed and other than doggie gastro a month or so back, she's healthy. But she feels the cold, and I'd like her to be a little heavier so if she was to get sick again, she can manage it.

Right now she eats supercoat dry food. She has a schmackos chew for breakfast. Bones are a bit of an issue - big bones make our other dog sick, shanks get stuck on her teeth. We do give her chicken necks. She gets skin irritations that are food related, but the supercoat seems to be okay.

So any tips on how to get her to gain a bit of weight will be appreciated.