I had this problem with my daughter too, she was born two months early and we actually had to wake her up to eat!
DO NOT add anything to her bottle unless directed by your Dr.!
Our ped recommended that we try to get her to eat within 30 minutes so she doesn't lose more calories than she takes in. At 30 minutes if she isn't eating we were to stop and let her sleep. We also were breaking her feeding flow to stop and burp her which we discovered was also a mistake. Yes they will spit up if you don't burp them but more than likely baby will pull away when too much air is in there and they NEEED to burp. Try letting your baby tell YOU when she needs a burp.
The things that we did until feedings were more normal/stable:
1. We changed her diaper first-she hated diaper changes so this would get her good and awake.
2. We stopped interrupting her feeding flow to burp her.
3. We actually fed her MORE often. We knew she could eat 4 ounces but one day had started falling asleep at two ounces. So we came back in TWO hours and got her to eat 2-3 more ounces. Overall big picture wise, the ped recommended that we try to get her to eat 24-30 ounces in a day, and this got her to take in more in one day than fighting to get her to eat for an hour and coming back again in 4 hours to do it all again.
4. We did experiment with different types of bottles as she was working so hard to get the food out of our Vent-aire bottles with #1 nipples that this made her too tired to continue. We eventually went with Avent bottles. I would advise you to buy 1-2 bottles and try it for 4 days before buying a bunch to see how it works if you go that route.
5. We kept a feeding log of how much, how long, burp (yes/no) and what bottle/nipple we used. This helped us identify patterns. When a little baby keeps you and Dad up all night, its hard to tell what works and what doesn't unless you log it. This also helps the ped make recommendations if you arent able to improve the situation by yourself.
I hope this helps! Good luck!