The well known Comfrey Leaves can help encourage a rabbit that is off feed to start eating. It also provides a source of hydration into their system. Moving, feed changes, weather changes can throw an animal off their feed. Word of caution, take care with angora/fuzzy breeds as the prickly side of the comfrey leaves attach like Velcro to their wool. I gave some comfrey leaves to a fuzzy lop once and she looked like she was sitting on a lilypad!
Strawberry and Raspberry leaves are said to be good for diarrhea. Just a few leaves daily for several days seems to help get them over it. Especially if warm weather is the culprit.
If an animal's droppings are few and they are still eating well but seem constipated, try feeding a few blackberry leaves or dandelion greens. Most will eat them when they won't touch anything else.
Be sure to give supplements conservatively. More is not always better or quicker in clearing up a problem. It may take a few days before you notice a difference.