Clearly this is an Inappropriate Elimination situation.
The Round Booda Dome litterbox that I believe you are referring to is very small. My cats can’t even fit in it. You need a much bigger litter Box. Actually you need two more!
Offer more than one box in different areas. You have 2 cats, you should have 3 boxes. One per cat plus one. I know a lot of people don’t like to have a lot of boxes… but sometimes that IS the problem. She’s a cat. She is picky. She may not want to poop in a diry box with the other cat. Which leads us to…
Keep the box super clean! If you are only going to have one box… it’s needs cleaned at minimum 4 times a day.
***** I believe if you take care of adding more and bigger litterboxes you will solve the problem.
Offer different "types" of litter pans and see if there is a preference to one. Buy cheap large roasting pans to test them in. Some cats hate certain litters or will flip their preference on a dime.
Check stools and make sure that they are not so dry as to hurt while having a movement. If they are... add a good healthy Organic wet canned food to the diet daily to soften the stools. Encourage water consumption with a water fountain filled with distilled water.
Check and make sure that she is peeing ok for sure. Make sure she’s making a full pee and not just a dribble. She could be straining to pee, and if it hurts... she would associate that pain with the litter box and then jump out to poo. If this is the case... she needs a vet asap! She could have a UTI!
Other Factors Contributing to Inappropriate Elimination
• Medical Conditions: Cats avoiding the litter pan should be examined by a veterinarian to rule out a medical condition. Laboratory tests will need to be performed in most cases, however, if a condition does exist, immediate treatment will help resolve the behavioral problem. Possible medical conditions include: colitis, inflammatory bowel disease (IBD), diabetes mellitus, hyperthyroidism, kidney or liver disease, or feline lower urinary tract disease (FLUTD). Medical conditions such as arthritis, FLUTD, anal sac disease, and some forms of colitis, which cause pain urinating or defecating may also result in inappropriate elimination
• Stress: Cats of all ages experience stress at some point in their lives (just like us). Stress can be a major cause of inappropriate elimination, and known stressors such as moving, changes in routine, or changes within the family structure (new members added or family members leaving home) can result in inappropriate elimination. Reducing these stressors or decreasing their impact on the household will benefit your cat and you, too.
• Box Location & Contents: Some cats may not like where their box is located; too close to their food or water, in a high traffic area, or on a different level of the house than where they spend most of their time. Some cats are very particular and will not defecate in the same box in which they urinate or go into a box which has been used by another cat. Most cats do not like a dirty litter box. Clean out waste from their litter boxes at least once daily, and wash the litter boxes weekly so that they don't decide to eliminate elsewhere in your home.
****************EDITED TO ADD*******************
I understand you are saying she is only pooping outside the box. Sometimes pooing outside the box means there is a urine issue though. Like a UTI, or once she pees in the box she deems it to dirty to use again because it's so small and she shares it with another cat so she poos on the nice clean floor.
Add atleast one more normal sized or super sized litter box and see if she stops.
Trust me... I live with 14 cats. I know all ablout litter boxes and snotty little cats getting fussy with things.
We can't even use normal sized litter boxes because we have huge mutant cats. LOL! We buy Clear Sterilite Storage boxes and cut a hole out for them to enter and exit. Any other *legit* litter box is just far to small for our cats.
And as you can imagine we have a lot of litterboxes.
We have no inappropriate eliminaton situations in our home... and the one time we did... I purchased the bigger boxes so our cats could fit in them and not have thier behind hanging out the box.
Get two more bigger boxes and you have a good chance of stopping this behavior.
~ Feline Wellness Advocate ~