What brett strain are you using? And are you using it as a primary strain or a secondary? If you're going all brett it's going to act like a sacc strain when used as a primary yeast. You can try adding Wyeast's brett lambicus which, when used as a secondary strain and given a long period and some complex sugar can impart cherry aromas. Another option is adding cherry juice at the tail end of aging and bottle directly after that. I feel like there are missing details though, like recipe details we dont know.