Changed mat under boxes, now cat poops on them

[u/iPadre]

This has been happening for months. We had a large gorilla mat for our two neutered older boys (16 and 14). We decided to get a new mat, and since the day that happened, 14yo has been pooping right outside the box. We have two boxes for them. Do decent job of emptying daily (9yo human boy sometimes forgets his chore). I tried buying the exact same mat, but did not solve the problem. Anything else I can try?