elderly cat defecation outside litterbox

We have a rescued cat approx. 13 years old who routinely defecate outside – usually next to – the litter box. She is fed grain – free & gluten – free food.This diet did improve hitherto watery stools. She does urinate in the box. Her appetite is good. She has been doing this for a long time. We have 2 other rescued cats who behave normally. “Constance” is the name of the problem cat. Thank you for any insights which you may have on this matter. RJP