Answer: Hydrangea macrophylla or bigleaf hydrangea flower on new shoots that develop on old wood. The most common reson for failure to bloom is winter kill. The temperatures get cold enough to kill the flower buds and/or the old stems where the buds were. Spring frosts can also kill flower buds. The years your plants bloomed were probably mild winter years and since the old wood didn't die back, it produced new flowering shoots. Pruning in fall, winter or spring can also remove flowering wood, so your Mother-in-Law was right. It's best to remove only the faded flowers, not the stems they were attached to.
These plants are reliably hardy in zone 7, somewhat reliable in warmer parts of zone 6, and unreliable in areas colder than that. If your plants are in a sheltered microclimate it can make a big difference as to whether or not they will thrive and bloom well for you. Finally, if they are in too little sun they may not bloom -- they do well in very bright dappled light all day or full morning sun. I hope this helps you troubleshoot and maybe correct the problem.
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