You do not say or whast kinds of hydrangeas these are or where you live or your zone but some possibilities: pruning the long stems in the Fall or Winter can cause this on mopheads; and Mother Nature can also cause that.

For mopheads, invisible flower buds usually develop near the end of the stems and 'back up' flower buds (located further down) kick into action when the ones at the end of the stem die due to watering issues or cold temperature issues. Winter protection could alleviate this problem. And yes, one plant could have this issue while the one besides may not.

Your bloom pictures were not close enough to tell for sure what kind of hydrangea this is but maybe a H. macrophylla mophead?

Normally, hydrangea pruning is usually not needed unless there is a reason. Stems that do not leaf out by -at the latest- late May, can be cut all the way down. For mopheads, which develop invisible flower buds in July-August and open in the Spring, pruning in the Fall or winter can result in no blooms. So you can end with your issue if you cut the long stems (say, in October) and leave the smaller/inner ones alone. Pruning mopheads is best done after they have bloomed in Spring (so you enjoy the blooms) but before the end of June or in July (just before they develop flower buds for Spring 2017). Down here in the South, I notice that pruning as early as July results in no blooms in Spring but, in the northern states, you may be ok. Mopheads, lacecaps, serratas and oakleaf hydrangea all do this so, try not to prune them now if yours are these kinds.

Paniculatas (Pee Gee-like of hydrangeas) and Arborescens (Annabelle-type of hydrangeas) develop invisible flower buds in mid to late Spring and the buds open as late as summer time. Pruning now would not affect their bloomage.

Now, I am not trying to say to never prune. You can prune stems that cross, you can prune dead wood, you can prune to get some more light "inside", you can prune plant growth that encroaches into sidewalks or safety problems, you can prune hydrangeas that grow so much that they touch other plants, etc. But normally, there is no need to develop a habit to annually prune.

Deadheading however, can be done at any time. Deadheading means cutting off the little "strands" that connect a bloom to the stems. I usually let Mother Nature drop the spent blooms but you can do this if they are still hanging around in Spring.

The following website explains differences between four of the most common hydrangeas plus it has good additional information. Start with a page that helps identify hydrangea types:

http://hydrangeashydrangeas.com/identify.html

Here is pruning info:

http://hydrangeashydrangeas.com/pruning.html

If you have never pruned them and the previous owner has never pruned them then Mother Nature may be killing the flower buds during winter. The way to confirm this is to give them some winter protection in the Fall and see how they bloom in Spring 2017. This can be an issue in cold zones and paniculata or arborescens-types of hydrangeas will provide your with the certainty of bloomage but the others will not unless you winter protect their invisible flower buds.

Wow, lots of information. Thank you. I live in Illinios near St Louis (zone 6b which is a recent change from 6a). I have not prune them but of course I don't know what the previous owner did. I will try to determine the type. They are nice and have blooms throughout the summer. So, I do feel lucky to have them.

Lovely!

Very nice, Clarence. They look like my mopheads did in late March (warm winter so they bloomed early). Enjoy as this is the best time in the hydrangea calendar. The 'next' best time in the hydrangea calendar is when the Annabelle-types and paniculatas bloom.... in late June and July.