Hibiscus buds are very prone to dropping, especially if your plant is under stress from too much or too little water, excessive fertilizer, thrips or other stress factors. Also, some varieties (particularly doubles) are more prone to bud drop.
The above cultural problems are the most likely causes. However if you feel they are not applicable I suggest you place an unopened bloom in a ziplock bag and take it to your County Extension Office to see if there are a lot of thrips in it. If so they can recommend an appropriate control product.
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