Hi and welcome
I tried the cat attract litter for 3 kittens I was fostering recently, as 2 of them werent using the trays consistently - I didnt find it made any difference! and actually I found the litter horrible to use, because unless you put a really good depth in the tray, the wee clumps go like solid rock and you need a little pickaxe to get it out LOL!! The two kittens would always use the trays for wees, but not always for poos.
I found that they used the trays more when I changed to Cats Best OKO plus litter, but they would still sometimes avoid (these 2 did have ongoing diarrhea, so I think that was the reason they sometimes didnt use the trays - and then it got into a habit). I would keep doing what your doing, encouraging to use the tray etc and cleaning up the areas she's used with a solution of biological washing powder/liquid and water, followed by a spray of surgical spirit when dry - to remove last traces of smells and hopefully preventing her from returning to those areas.
Would love to see some pics of your Devons!