Another thought; if you've made the item yourself, and I've used my own without issues, try going through the process of making it conforming. I do that with items I create for dynamic use only, it saves on a pose series to drape some of them; not always but often. Presuming that works without issues, then it may drape well leaving it fitted to the figure; and if not, just 'unfit' then try draping. I'm wondering if the item you're using has issues, then those might also cause Daz to object when you're trying to fit.