Hi Natalie and welcome to the group. Congratulations on Hollie's birth.
My youngest daughter Gemma is now 7 months old. She was born at 33 weeks, 1800g. She was in NICU for 2 weeks as well. Luckily we had no reflux problems, but we dealt with poor weight gain for a looong time. You may see an improvement this month. With G, once she was 3 months old, she turned a corner. It was as if she woke up & started feeding & growing. However, really good weight gain did not happen until 6 months, once I started her on rice cereal mixed with formula. As long as your doctor is happy with her weight gain, don't worry. Everyone tells me that it will creep on over the first year or 18 months.
Lots of people have problems with reflux anyway--without prematurity. But I know it makes everything much harder.
DS no 3 also had terrible problems with weight gain (he was not premie). It took a full year before he stayed on the 25th percentile. It seemed that nothing we could do could make it stick to his bones. He's now a tall, skinny 2 year old. It is frustrating, but time will help a lot.