Sticky uneaten cecotropes occur due to an imbalance in the microbial gut flora. Some rabbits are more prone than others. It isn’t a serious health problem, but it’s inconvenient for both you and the bun.
The cause is most often dietary. You can cut out all greens and treats, but unless your bun is a very good hay eater, there are risks with only serving hay and water. If the bun isn’t used to eating lots of hay, it can have trouble taking up the nutrients from the hay.
Digestive breakdown of hay is dependent on the cooperation of many different gut microorganisms. This bacterial balance needs time to establish, and meanwhile, the rabbit can essentially starve even if it eats the hay. Another problem with a hay-only diet is that some buns who are not used to hay can refuse to eat the hay and for that reason go into gut stasis.
You can cut back on pellets. You can taper the amount down gradually. If your bun isn’t fond of hay you can try other types of hay or add some dried herbs to the hay to make it more interesting. You can use a bunny-appropriate probiotic to help the gut microbiome.
If the bun’s behind isn’t very dirty, you can do a cornstarch dry “bath”. You massage corn- or potato starch into the poopy bits of fur, then pick and/or brush out the now crumbly clumps of poop. This method is very gentle on the bun.
If the bun’s butt is very dirty, you can do a butt bath. Its best done in a plastic wash basin or similar, placed on the bathroom floor. (Its safest on the floor in case the bun decides to jump out). You fill the basin with a couple of inches of lukewarm water (try the temp with your elbow, as if you were to bathe a baby). Then place the bunny in the wash basin – I find it good to let the bun stand with its fore paws on the rim of the basin, because that gives the bun a sense of control. Gently rinse out the sticky poop using your hand. If you need to use shampoo, use only a few drops of a mild, perfume-free soap or schampoo.
Its important that you dry the bun after so it doesnt get cold. Microfiber towels are very good for this, but a terry cloth- or a soft dish towel will obviously work to.
If the sticky poop persists, it is wise to do a vet check up.