Gall Midges - Hibiscus Buds Turning Yellow and Dropping
If your hibiscus buds are turning yellow and dropping before they open, pick a dropped bud up off the ground and open it, especially if it is freshly dropped. If you see tiny little yellow larvae (use a magnifying glass) those are gall midge larvae.
Pick up the dropped buds and place them in plastic bags for disposal. Use imidacloprid soil drench to kill the larvae that make it to the soil to pupate otherwise they will just mature and then the cycle repeats.
Not sure how far north you find this pest but here in South Florida we have them.