What can I do to stop my 11year old cat from pooping on my carpet?

My female cat boo is 11years old( fixed and declawed) I have 4 other cats, 8years, 5years, 1year, and 4months; all are declawed and fixed too! I have 6 litter boxes in my house and the oldest cat, the problem/boo goes in the litter boxes once and a while but still poops in one specific spot! She has arthritis and I give her pills mixed with soft food but that makes her do nasty squishy poop! I don't know what to do I won't get rid of my other cats so should I put her down or can I find a good home for her?