While Heptacodium miconioides are recommended for full sun and generally do well in that location in the New York area, they benefit from some light shade in more southern locations and may become stressed and tired at the end of a hot, dry summer in zone 6.  If you are experiencing problems, your tree is possibly somewhat stressed, or still young and not yet fully established.  These are most likely to be its problems though growers do report variation in sepal quality and color among specimens.

Other preferred cultural conditions include a richly organic and slightly acid soil, which is why these trees are placed in light conifer shade in hotter areas. Flower and fruit production may also be affected by the sequence of hot months followed cold months and dramatic fluctuations in winter temperature from cold to warm can affect the following season’s product.  Be sure that you don’t prune unnecessarily in the spring.  The buds on this plant emerge so far in advance of actual bloom that a gardener can be fooled into pruning too late in the season.

It sounds as though there is no reason to believe that your Heptacodium is unhealthy, though some additional water at the driest parts of the summer may be in order. Dress your soil over the root area in the late winter with a rich organic top coat and feed with a  balance organic fertilizer in early spring.  Hopefully the coming year will yield a colorful result for you!

More info: http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=k450

Hope this is helpful.

Courtesy of the NYBG Plant Information Service