Hi, I'm no expert but I wrote a post similar to this when dd1 was 26 weeks( 12 corrected) she was born at 28 weeks gestation 4 years ago. What I did was push to 4 hourly feeds as she was gaining slowly but gaining. She didn't automatically take more but was just about taking about 20 ounces a day. Also I have a 5 month old and from around 3 months she was definitely less interested in feeding and more interested in what is going on around her. Again i moved to 4 hourly feeding and again she prob only takes around 20ounces a day.
What I'm trying to say is providing she is gaining weight and feeding regularly try not to get too concerned over volumes. My health visitor echoes this by saying all you can do is offer the feed- how much she takes is up to her. Obviously if she starts to lose or not gain then you may have to rethink x