Answer: Although you don't see any physical signs of insects, I'll bet if you closely inspect the unopened buds on your plant you'll see tiny little holes in them. What you describe sounds like the work of a larvae called a bud worm. The adult lays an egg on the outside of the unopened bud and when it hatches it drills into the bud and munches on the petals and ovary. When it has had its fill it exits the flower bud. The unopened buds will look perfect (except for the tiny hole) until they begin to open up. But without all the necessary parts it will fail to open. There's not much you can do to stop this problem (the bud worms are safely enclosed in the protective bud). However, you can reduce the pest population by carefully inspecting the unopened buds, pulling off and discarding any with the tell-tale hole in the side. This will at least control future generations.
Best wishes with your hibiscus!
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