Dropping hibiscus buds
 

Why do the buds on my hibiscus turn yellow and drop off about two days
before they should open? The other buds are okay and look a healthy shade
of green but when they get to a certain size -- and just as I can see the
color of the petals inside as it begins to unfurl -- they turn yellow and
either never bloom or just drop right off.

Any suggestions?

Giselle