There are a variety of causes for weight loss in rabbits. For a rabbit that is eating pellets and hay without any problems, and is eating a lot of hay, then most likely causes for weight loss in rabbits that still have a good appetite and are getting plenty of food, are worms, hepatic coccidiosis, e. cuniculi, organ failure, infection, cancer, and a variety of other health conditions related to old age. Uterine cancer is especially a high risk in unspayed female rabbits. In rabbits that are not eating normally or their normal amounts but still behaving normally, the most common cause would be dental problems.
http://www.rabbitnetwork.org/educat...abbit-health-articles/weight-loss-in-rabbits/
There are otc medications that can be used to treat the various parasite causes. Other health conditions would need vet treatment, and even then it's possible there may be no way to correct the problem. Dental problems can usually be corrected, and if minor sometimes diet changes can fix, but if too advanced would need a vet to fix.
Is your bun still not eating hay very well, and how much would you say she eats in a day(a pile the same size as her body or less?)? Are you limiting her pellet amount each day, and if so what amount, what is her weight, and what type of pellets are you feeding? If your bun is getting a limited amount of pellets each day and is still not eating hay very well, that would explain the weight loss and points to dental problems as a likely cause. Even the smallest molar spur can affect a rabbits eating, and with your vet not sounding very rabbit savvy, it is a good possibility the vet missed dental problems as the likely cause. Plus it can be hard for a vet to get a good look at the teeth of a conscious rabbit and sometimes a GA is required for the vet to get a good look in the rabbits mouth.
If you think that dental problems might be the cause of your buns weight loss, you will need to get more food into her until you can get the dental problem corrected. If she's still not eating hay well, then this will need to be done through increased pellets, adding in hay pellets(plain hay pellets used for horses) to the diet, or feeding critical care daily(which is ok to do). If you are going to increase pellets though, how exactly this should be done depends on if your bun is still eating pellets really well and what type you are currently feeding. Some types aren't meant to be fed in larger amounts and could potentially be harmful if overfed.
If you need to get her back to a vet at some point, here are some rabbit vet lists you can look at, and hopefully find a better vet near you.
http://www.trianglerabbits.org/health/vet.html
http://www.rabbitsonline.net/showthread.php?t=10097