What Causes Feeding Intake Problem In 4 Week Old?

My baby niece is now 4 weeks old. since day 10 she has been hospitalised twice. First due to rapid weight loss and poor feeding, which was attributed to a viral infection, she was discharged and 2 days later re-admitted due to poor feeding. The main problem since approx a week old seems to be that she cannot take more than 3 ounces of feed without vomiting or falling asleep. Whilst in hospital she has been fed through a tube and whilst they feed little and often this worked ok, however each time they try to up the volume and feed at longer intervals the same problem re-occurs. The doctors have checked bloods, saliva, water sample etc and have now put her back on 2 hourly tube feeds to ensure she is getting sufficient feed. My sister is convinced that there is something internal that is preventing the baby from taking more than 3 ounces, but the hospital seem reluctant to investigate. They are treating the baby for reflux and have 2 days ago changed her to a non-cows milk feed in case it is lactose intolerance, however the same problems still seem to be occurring - vomiting and sleepiness. The doctors asked if they could feed the baby for 24 hrs (bottle) so they could observe for themselves, and I assume to see if there was any issues with mums and dads techniques, but the sames problems occurred, hence she is now back on the 2 hourly tube feed. She is not putting weight on, however is not losing weight either. Any suggestions as to what could be preventing an otherwise healthy baby from properly feeding?