Bud drop from hibiscus - need advise?

I have a rooftop gargen with 18 hibiscus plants (in pots). 16 are Ok and flower well. There are 2 plants which produce lots of buds but never flower. The buds turn yellow before they bloom and fall off. I have tried adding extra manure but no result. Please advise.
Bud drop from hibiscus - need advise?
The Redneck listed good ideas. We've also seen buds fail to develop in retail plants when they've been neglected and under watered. Since you're having luck with all your other plants, I'd be looking for the differences between them and the sickies. Are they different varieties? Are the ailing ones in a place where they get more wind than the others, or more or less light? Were they potted a season before the others or with a different potting medium? Do any neighbors use the garden, too? Are they using the pots as ashtrays, or as kitty litter boxes? There's something different going on with these 2!
Reply:Bud drop can be caused by insects, overfertilization, poor drainage and excessive water..