Answer: In my experience, clethra does best in moist to wet soil on the acid side. It prefers soil well supplied with organic matter and while it will grow under almost any lighting condition from sun to shade, it blooms best in part to full sun. In other words, it will tolerate a slightly drier soil if it is in more shade, but then it blooms less. For the record, any pruning should be done in very early spring.
Assuming your clethra is getting enough moisture, my next thought is that it is oversupplied with nitrogen. Sometimes a shrub will react to extra nitrogen by refusing to bloom -- instead, it grows lots of lush foliage. If you have been fertilizing your clethra, I would stop for a year and see what happens.
Unfortunately, since clethra blooms in late July or August on the wood formed during the current year, it may take a year for a revised water or fertilizer regimen to take effect. Good luck with your shrub!
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