Hydrangias won't bloom.

I have several hydrangeas and all bloom quite well, however I have five Nikki Blue plants that dont bud until late September. I feed them 10-10-10- fertilizer and acidify em as I do the other plants. I realize they bloom on old wood and I don't prune them down very low. The stems are coming from old wood. They did bloom well last year but didn't the year before nor this year. Help please.
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