There is a litter which has a great track record in converting cats with litter box issues. (meowcatrescue.org)
In the case of litter box issues or if there's any tension between cats then the rule changes to having one more box than you have cats. (catbehaviorassociates.com)
Some cats develop litter box issues because they're stressed by the inability to defend themselves. (kittycatter.com)