from what I've read, and from my own terrible personal experience, a rabbit loosing weight is a potentially very serious issue. I think bunnies are in general very robust hearty animals...there is an exception...their gastrointestional system. a healthy bunny is a hearty eater. If they are not eating enough to maintain weight, something serious is going on. They will slow down on eating if they have teeth problems. (Their teeth are normally always growing and normally they will grind down their teeth chewing...hopefully hay and not furniture.) If their teeth are not being ground down, the teeth will elongate and make it painful for bunny to eat. Also, the continually growing teeth can also start to push upward into their skull causing severe problems. If they have chewing problems, the start swallowing inadequately chewed vegetation into their primary stomach and then it can clog and block off their gastro system...GI Statis. Bunnies cannot vomit. Once their primary stomach is blocked they are going to die without medical help. When the primary stomach is blocked, their secondary stomach, the Cecum will be emptied. The cellulose to glucose transforming bacteria in the Cecum will then die. Without this source of glucose, this is why the bunnies are facing a life threatening situation.
Cliffs Notes: If your bunny is loosing weight, and the bunny has adequate food available..but still loosing weight...see a vett soon.