Answer: This could be partly to age and partly to the length of your growing season being so short -- you are really at the north/cold end of their growing region. I hope they are planted in full sun as that's what they need. Also, limit any pruning to removal of winter damaged tips each spring. They bloom on new growth of the year, so summer pruning would limit flowering. I would allow a few more years for them to mature before worrying too much.
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