How can I get her to eat more??

I recently found out that my 11 week old hasn't been gaining weight and has lost a tiny bit. So, I've been trying to get her to eat more and longer, but I think she got into not having an appetite while she was sickish a week or so ago and won't eat for long. I'd only been able to feed her for about 20 minutes and apparently that's not enough. The past couple of days, I've managed to get her to eat for 30-40 minutes everytime, but she seems to fight it for some reason. How long should she be eating each feeding and how far apart should the feedings be? I am exclusively feeding, by the way. I just need to know a way to get her to want to eat for longer periods of time.