Litter pan cleanliness - cats have an exquisite sense of smell, and just as you would not want to use a dirty port-o-potty, your cat does not want to step into a smelly litterpan. Scoopable litter should be scooped at least 1-2 times daily and changed completely on a regular basis.
Open concept - cats(being not only hunters, but prey creatures) prefer to be able to see what is around them while using the litterpan. Covered litterpans trap unpleasant odours inside the pan as well making it less attractive - remove the lid size does matter - small tight spaces or deep pans that are difficult to step in and out of are unattractive especially if your cat is large or is getting older and has some mobility issues (this maybe the first indication that there are mobility issues).
Numbers - it is recommended to have one pan/cat plus one in multicat households.
Location - pans should be available on all floors of mulit-level homes, in open areas (not in a closet) away from loud machinery (washers, dryers, furnaces) and from other pets (that young puppy)
Substrate - the type of litter that you use may make the litter pan more or less attractive to your cat - remember that in the wild they prefer a loose, sandy soil for "doing their business".
Fragrances - avoid litters that have strong perfumes/smell
A complete physical examination and urinalysis are recommended. Blood tests, radiographs and/or Ultrasound may also be recommended.
Possible Medical causes:
You may need to rule out causes such as: