I wish I knew, but I don't germinate seeds if any kind. I have zero skill on that front.
My knee-jerk reaction is that they're waiting for real sunlight or even just water temperatures.
Have you tried just direct-sowing them outside...in pots or in the ground?
I bet someone on the Hibiscus forum could shed some light. Try this thread, as it's all about hardy hibiscus.
https://garden.org/thread/go/1...
