If your daughter is having breathing difficulty, that could make her burn more calories. Do you have a machine that monitors her oxygen saturation? It's called an oximiter? This is a great machine, because you will know exactly the right level of oxygen she needs at all times. It's hard to think the oxygen is going up, but trust me, she will be off of it before you know it.
Another thing, are you certain she is getting enough to eat? Is she on a high cal preemie formula. You can get formulas that go up to 24 cals per ounce, and you can even mix formula to higher calories per ounce if you know the right measurements. Max was on 30 cals/ounce to help him gain. Best way to know if she is getting enough is by output. She should be having 6-8 wet diapers a day and at least one BM (although skipping a day or two here or there isn't a problem and is often common with young babies).
I hope you find the answers! I know how hard it must be to not have your baby growing!
DH, John (36)
Max, 1/20/03, 25 wks, 534g. (IUGR), severe PE