Irs2015, we understand. We were there too. DS was born at 5 lbs 6 oz. And in NICU for months. Gaining weight the first year was hit and miss. We worked with the nutritionist but it wasn't enough.
We finally hit on Ovaltine. He didn't like supplements but loves chocolate milk. We didn't want him drinking something without any nutritional value so we tried Ovaltine and it was a hit. Some use Carnation Instant Breakfast and similar potions, but chocolate flavored Ovaltine (not Malt flavored) was a hit. We put it in a sippy cup and never let it stay empty for more than a few minutes. We used 4tbs instead of 2 but see if that's too strong a chocolate flavor for her. DS wound up drinking 4-6 sippy cups a day and at 240 calories a sippy cup it's exactly what he needed. When we switched centers at 2 years old, the docs and nutritionists were shocked at how great his perect chubby weight. He was chubby, pink, and healthy and happy.
Now I will add two other techniques we used. First of all when you have this kind of problem, don't be so picky about what food you use to up her weight. Whatever she likes, give her a bunch.
The next thing we did that really contributed to our success, was to feed continually. Even with sitters and later daycare, the instructions were to feed, wait 15 minutes to 30, offer him a sippy cup, wait 15 to 30 minutes, offer him a snack, wait 30 minutes, feed lunch. Wait 15 minutes offer a sippy cup, etc.
Remember - never, ever force, never ever never ever. I don't care what it is or at what age, if you force, you build resistance not compliance.
Finally, our last strategy was to not accept his indications that he was full at a meal without one more go. Most people stop eating when they've had enough especially of what's being fed. If it's his favorite food, he will fill up and quit. Just take the bowl away without any attempt to get her to take another bite and smile. But then I would head to the cabinet for a DIFFERENT food. Pull something out.....chips, dip, applesauce, juice, pudding, crackers, ice cream, something that had not been served. I might take a bite myself, for me, not him. When he sees me eating something he likes, ask if he would like some. Usually the answer was yes. It was something different from the meal, so he wasn't full of THAT. If he said no, I honored that without any additional prodding or remarks.
DS has maintained over 50% - typically 75% - BMI since then, despite being born at less than 10% and staying very low the first year.
Let me know how she's doing and feel free to ask any questions.
Blessings!