We’ve been having a wet summer (until now that is) and, if you have soil that holds water well (normally a good thing) then the plants may under stress from that and bud drop is a response to that. Both the types of hibiscus you have have deep root systems so rarely need extra watering except in the driest of years. Stop watering. It has also been humid so it is possible that your hibiscus have a fungal disease called botrytis. Earlier spraying with a fungicide might have prevented this but probably won’t help much now that it has taken hold. There are also thrips, a very small insect that may go undetected due to their small size and light color. Take a close looks at the flower buds to see if there are an small white kind insects on them If so, a spray if insecticidal soap once a week for three weeks should knock them out. Thrips are sucking insects so there are no holes or obvious damage but the damage they cause can cause the buds to fall before opening.