Steel cut oats have worked great for Ellie because they are very high in calories.
Are you feeding canned food?
If they don't have medical problems and are very underweight, I would feed them when they are hungry. Ellie's vet says when she is hungry, let her eat.
Ellie was between a 2 and 3 on the body condition scale (5 is perfect and 10 is way too fat). She is probably up to a 3 or maybe 4 now, so she is gaining.