Answer: I can't tell from your description of the problem so I'll suggest a couple of possibilities:
If your plant looks otherwise healthy, the bud drop could be caused by gall midge. Place some buds in a sealable plastic bag. After a day or two, you may see the very small yellowish worm-like larva on the plastic. Control the midge affecting buds with Orthene sprays as recommended on the label.
Shrub Althaea (Rose of Sharon) is subject to a disease called bud blight, caused by Botrytis. It's possible that your plant has this problem but it's impossible to tell without seeing it. You might want to take a sample of the problem to your local Extension office for positive ID and registered controls in your gardening region.
Hope this information is helpful.
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