The most common cause of bud drop in hisbiscus is over feeding. Is this possible in your case? An alternative could be the variation in temperatures between hot and dry and cool and wet. It may be that this particular variety is susceptible to these changes.
I would suggest that if it is dry you water it on a regular limited basis and see if this makes a difference next year.
The only other possibility is some insect damage to the buds, such as a mite, but it would be surprising if this didn't affect your other hibiscuses.

hello diannet, i have been a member on this site for about 3 years and the question you have asked was one of the questions i first asked, i have 7 hibiscus in my garden, 3 are standards and 4 bush type i have grown from seed, one of my std,s which is now 6 years old, has a million and one flower buds which form perfectly and just start to show the colour (burgundy red) then thats your lot they have never ever opened yet, the answers i received where wrong type of soil,so i moved it to a more acidic part of my garden, not enough or irregular watering, thrips infestation,done all the usual checks none evident,all my other 6 flower perfectly, just to prove a point to myself one of the plants i grew from seed i planted within 2 feet of this std and now in its 3rd year as expected it has loads of fully formed blooms but the std ! plenty buds no flowers,i have researched this all over the web and it appears to be a common thing with certain hibiscus, never had a problem with any of my others even ones i have given to friends, I know this does not help you but at least someone answered you,if you ever work it out would love to hear from you,
regards, ogon,
p,s, I live in the wigan area and watering is not the problem ha ha,