Le stai annaffiando o fertilizzando? Sono stabiliti, cioè 2 anni o più? Se è così, non li annaffiate e non li concimate. Inoltre, sono vicini al prato? Se usi un fertilizzante ad alto contenuto di azoto sul prato, anche i cespugli lo ricevono e non fioriscono bene, se non del tutto. È un ibisco tropicale o resistente? Ho diversi hibiscus resistenti e non ho mai visto il problema. Una volta ne ho anche spruzzato uno con il Roundup per sbaglio e ha fatto ancora bene dopo averlo lavato per 30 minuti, LOL! Se è in una pentola stai annaffiando molto! Innaffiare solo quando il terreno è asciutto!

Hibiscus has been a very difficult plant for me, every 10 days it will have some disease or the other. This issue you are talking about has been faced by me too a few days back wherein my plants had a lot of buds, but before they could flower buds will turn little pale and fell down. I was wondering if it has been any nutrition deficiency till I popped open one fallen bud and saw little pests inside, they were too small.

But this is not always the case, you can follow this checklist to reach to the root cause of the problem:

1. Check if the plant is healthy-looking, look for the color of leaves for eg, if it is dark green and upright, that rules out any root problems because this can also cause falling buds issue.
2. Write down your watering schedule, it should not be too frequent and it should not be done with too much gap. Too little watering and too much of gap can also cause this as the plant goes in shock. Too much of watering is an even a bigger problem, so be considerate.
3. If there has been a recent change in the place of Hibiscus plant, give it some time to adjust in a new environment. for eg, if you have placed the hibiscus in Southern part of your house which receives lots of afternoon sunlight to Nothern part which receives morning to noon sun, this change in the amount of light received will also cause a plant to go in shock and display this behavior of dropping buds.
4. Change in daylight time or heat, too much heat will cause dropping of buds if you have noticed this behavior with the start of summer months, then that could have been the issue.
5. If all these are not applicable then your plant is under attack by some pest, maybe aphides or midge larvae, best and organic treatment for any kind pest attack on hibiscus is Neem oil treatment, few drops of neem oil with an almost equivalent amount of handwash or soap and 1 liter of water, mix it really well and by the use of sprayer, spray this solution onto every leaf, stem, and bud of the plant. Do it in the evening or early morning and repeat every 7 days to 15 days. You can also use OTC pesticides also, I don’t use or recommend these chemicals.

Hope this will help. Thanks!