My response is kind of late, but maybe it will help anyway. Plantings next to the house foundation have been known to have problems at times, so it may not be surprising that a hydrangea in such a spot might also have problems. For one thing, foundations provide some protection against the rain--which could mean that the plant, overall, is getting less moisture, even though you are watering it the same as other plants that also get moisture from the rain. In other words, watering that hydrangea "the same" as the other ones may be the problem--because it needs MORE WATER to compensate for some missing rain water.

It can also be warmer next to a house foundation--another reason a hydrangea in that spot might need MORE WATER than the other ones do.

Don't know if this will solve your problem, but next year, I'd try extra watering and see if that helps.

Kate