I would get one of those soil kits that tests for too much nitrogen and determine if somehow you are accumulating too much fertilizer where that non-bloomer is located. Sometimes lawn fertilizer, combined with the hydrangea's fertilizer combine and become too much so you get no blooms.

As we get into Spring 2012, I would check if the shrubs are developing any flower buds or if the flower buds are developed and then are being eaten. So observe if pests like to congregate there... pests like deer, squirrels, etc congregate. They could be eating all of your flower buds. However, I would expect them to also munch on the other two NB. So I am not sure if this is THE cause but it is worth checking anyways.

If you are not getting any flower buds then you may have an environmental problem. It is too cold and the buds dry out, for example. Or it is too dry during the Fall and-or Winter and the plant aborts the flower buds. NB should be hardy on your zone 6 so I would check if this one gets less water.... say because the sprinkler waters the other two fine but does not reach well enough to this one NB.

Dense shade can be a problem too but if all three were in dense shade, I would expect them all to be affected.

Just wondering, have you ever had a formal soil test done?

Thanks for the suggestions! I've never had a soil test done but will certainly do so at your suggestion. I have wondered if the soil in that particular shrub has changed over the yrs.

Honestly i don't fertilize anything around our house. There is no shade and I live in town. No deer - lots of squirrels :)!

I saw a few buds on this Spring and then nothing happened. Meanwhile all of the other ones in the same row bloomed like crazy.