Help! My cat won't poop in the litter box. What should I do?

Hello, sorry to hear about Om. There are a couple of things that can contribute to this. Primarily have 1 litter box per cat and then an additional one is a must. Having the litter boxes in a private area also helps. Finally, trying different types of litter is another thing to consider. Here are some tips on litter box use what may cause inappropriate eliminations. Hope this helps and best of luck.

Feline inappropriate elimination is a common behavioral problem. Many owners may become frustrated trying to cope with this problem. Help is available. Because inappropriate elimination is associated with an increased risk of relinquishment to shelters, treatment should be sought early on. Contact your veterinarian or local animal behaviorists for additional help.
What is inappropriate elimination?
Inappropriate elimination is the deposition of urine or feces outside the litterbox. Cats displaying inappropriate elimination may either be eliminating in the wrong area, or exhibiting marking/spraying behavior as well.
Inappropriate Elimination vs. Spraying / Marking?
Urine marking is a distinct problem separate from inappropriate elimination. These two behaviors are similar and can often be confused by owners. Urine marking is most common in intact males, but a relatively large proportion of cats will display this problem.
Reasons why your cat may be eliminating inappropriately:
Medical: A number of medical conditions may contribute to and intensify house-soiling problems. Kidney, liver, and thyroid diseases can cause increased drinking and increased urination. Inflammation of the urinary tract can result in painful or frequent urination, or inability to urinate. Once a cat associates the litterbox with urinary difficulty, he or she may be reluctant to return. Animals may also experience age-related changes (arthritis, loss of vision or sense of smell, senility) that alter normal litterbox habits. A complete physical examination, plus other recommended diagnostic tests by your veterinarian is necessary to rule out and treat medical problems.
Substrate, Litterbox or Location Aversions and Preferences
Cats may find fault with the chosen litter substrate, litterbox location, and/or the litterbox itself. Dirty or artificially-scented litter is often distasteful to cats. Litter texture may also be unappealing– most cats prefer fine, sand-like clumping litter, while some prefer coarse, non- clumping litter or soft substrates, such as towels. There are many reasons litterbox location may be offensive, such as close proximity to highly-trafficked areas, feeding areas, loud noises such as washing machines and dryers, and other pets (particularly if they terrorize or ambush).The actual litterbox(s) may be too small, insufficient in number, contain harsh odors or aversive plastic liners, or have sides that are too high– a particular problem for kittens and geriatric cats. Cats with a preference for an inappropriate substrate or location may still continue to use their litterbox, but inconsistently.