So basically my DD in 9 months old (6 corrected...

So basically my DD in 9 months old (6 corrected). She has always been a TERRIBLE feeder and only has between 2-4 bottles per day of 100ml. So not enough I know, but we are under the watchful eye of a nutritionalist, shes extremely underweight too. The bottles we use are the NUK wideneck ones, and there are two choices of teats: Stage 1 which is for breastmilk or water and Stage 2 which is for formula. So my problem is: Shes never had the stage two teat as her mouth was FAR to small (preemie) and I felt the formula came through fine, and now I can't get her to have the next size up teat, it still seems wayyyyy big in her mouth!! (Shes about the size of the average newborn, to give you an idea). I'm not sure what to do, and think that the milk's not flowing fast enough, which infuriates her more! Should I change the type of bottle? Or persist??? Help!!! TIA :-) (Sorry for the novel!)