I don't have hibiscus but I'd imagine it's a more floral, maybe almost perfumey favor. Some flavors you really taste most on exhale, sometimes
through one's nose. The other possibility is you may not have steeped it for long enough, and/or it's one of those flavors that shines as an accent note, not a standalone. I would think 15% hibiscus would be pretty nasty, and if you haven't steeped it yet, well, steep some at a lower percentage and/or save your 15% mix (it is going to be overflavored, probably) to add as an accent note to other flavors. Some flavors are so subtle, you may not really taste as much as they tend to work best as accents though, and I am considering this may be the case here.
I am prepared to be wrong about all of the above however, I am merely speculating. Good luck!
Anna