Why won't my clethra bloom?

I have 2 Clethra plants that I purchased and planted about 10 years ago. (at least) The local nursery from which i bought them suggested these shrubs for my shady, acidic site in zone 5. One should have white blooms and the other should have pink blooms. They have grown in size and are nice and full but have NEVER bloomed. They are planted in an area that is not wet, but also not too dry. Does anybody have any ideas for me? I am considering replacing them with a different shrub but if they bloomed, they would be perfect. Thanks in advance.
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JanB wrote:

Try cutting it back before you give up on it. Flowering occurs on the new growth of the curren season. It's too late for this season but try it for next year. This shrub does OK in most any condition. Best performance occurs in partial sun to partial shade in moist to wet acidic, fertile, organic soils. HTH
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