Agree with gumby - the most common cause is too much water. Can't control the rain you may get but you can cut back on watering and make sure the plants are in well draining soil. Without more details or pic of the plants in question, that would be the best guess. Dave

Endless summer hydrangeas are macrophyllas. IOW those who usually bloom on old wood. They, however, are mutants who bloom not only on old wood, but new wood. They became popular for folks not only to extend the bloom time, but the zones in which a macrophylla could be grown and reliably bloom. If your's isn't blooming yet, it is probably because you pruned off the old wood it would be blooming from. Another reason for low bloom response is the buds set on the old wood were frozen out in a hard winter. So, if you had severe weather last winter, they may not have bloomed on the old wood anyway. We wait until spring to prune our Endless summer and any other macrophylla to see how much old wood survived. On our standard bushes, we don't expect bloom. On the Endless Summer we expect some bloom when the new wood is mature enough. You should be getting some bloom later on your new wood.

Too much water can cause the same signs and symptoms that too little water can cause. If you don't see wee bugs on the leaves that means the problem is not Aphids, Mealybugs, Scale, or other similar insects. I would check the soil first.

I have a bunch of first year seedlings and was surprised to see that many of them went dormant just like my mature plants. What you are describing sounds just like what happened with mine. The size of the seedlings didn't seem to be a determining factor either. There were an equal number of 6" as well as 15" seedlings that dropped their leaves. Karyn