How much does she weigh? How old is she again? Oh, and what breed is she?
The usual guideline for pellet food amounts is 1/4c per 5lbs of bunny...but that also depends on the breed and the age. For instance, if you had a bun particularly happy about eating (well, piggy-ish), you'd want to give the bun a little bit less per day. Also, certain breeds put on weight different ways, and some breeds need alfalfa pellets longer, as they grow for longer, etc.
It sounds to me like you might want to change her over to a Timothy pellet. I had to do this with Maisie when she was younger (at about six months, if memory serves), because of the same problem. Alfalfa pellets have more protein than Timothy...and some buns just have a hard time processing the excess protein, and then stop eating as many of their cecal pellets because of it. (You'll be able to read more about cecals, also called cecotropes, in the rabbit poop link Naturestee gave you.)
You mentioned she needs to gain weight. You can still slowly switch her to Timothy pellets (I'll give you a guide on how slowly to switch in a moment), and add some oats to her food daily, and it'll help her to still be gaining weight.
When switching buns' food, you'll want to do it gradually. Here's how I usually switch my buns' food when I've had to:
FIRST WEEK: 3/4 old food, 1/4 new food
SECOND WEEK: 1/2 old food, 1/2 new food
THIRD WEEK: 1/4 old food, 3/4 new food
FOURTH WEEK ON: ALL new food
So, there ya go.
Also, be sure to check out the thread on
Pellets in the
Bunny 101 section of the
Lagomorph Library. Lots of helpful information there. The
Carrot Cafe site also has really wonderful information on dietary topics.
Let us know if you need anymore help, ok?
Hugs to you!
Rosie*