When we got the Arabian my daughter leases, she was underweight. The ranch she lived at told us she was always a hard keeper and that she'd recently lost weight when they were switching feeds. So she was pretty skinny when we got her ... not as skinny as your mare, though, I don't think. She was about a 4 on the 1-10 weight grading scale.
Our mare can't chew very well because she's 29 and has teeth problems, and she also wasn't absorbing nutrients very well. We give her probios to make her gut function better and absorb nutrients better. Her daily ration is divided into three feedings, but her daily total is: 6 lbs. Of senior feed, 1 1/2 cups oats, 3 scoops of Fat-Cat which is a weight gain supplement highly recommended by users at the smartpak site. Once a week I give her 1/2 c. Omega Horseshine instead of the Fat-Cat to get some omega 3's in her for her coat. We mix this all into soaked beet pulp for the carbs since she can't chew the hay very well, but your younger mare shouldn't have that problem. She *might* need her teeth floated, though! Some horses need it done more often than others, and if her teeth are really bothering her she may not be eating as much as she should.
I'd suggest reading the reviews for the Fat-Cat and seeing if that might be a good option. I think it is better than the Dumor. And I would definitely add probios to get her gut in good shape (the powdered stuff is more convenient than the tube). Wormer can really kill off the good bacteria in the gut, so the probios can help there. Fat-Cat - Horse Weight Gain Supplements from SmartPak Equine
Lacy looks really good now. She's put on just the right amount of weight and I don't worry about her anymore. I do feel like 3 scoops of Fat-Cat is more than most people give their horses (I think 1-2 scoops a day is average), but again, Lacy is old and can't eat enough hay.
Hope this helps some!