Its root system is never going to be the same as those of other of the same varieties so maybe it has not gotten established yet even if they all are the same cultivar. Oakleaf hydrangeas are very sensitive to root rot and this area looks "busy" with a lot of nearby stuff competing for nutrients and water. Not sure if the foliage has displayed soil moisture issues before; it looks fine now. I assume you test the soil moisture and not water if the top few inches are either moist or soggy/wet under the drip line? At two years, I would definitely ignore it as three years seems to be a good time to usually start worrying instead. Winter damage in the form patchy frost is known to affect plants selectively (only this one, not the one next to it, a few after, only the top, only one side, etc.) so I wonder if the leaf buds got hit. Some of the western suburbs of Chicago are at the limit with z5a close by. I assume that you stop fertilizing around the start of July? You do not want to fertilize too early or keep new plants actively growing as your average date of first frost arrives in the fall. The average for western Chicago varies but is around the 1st-2nd week of October (for Aurora) so one should apply a slow release fertilizer as late as the 1st-2nd week of July. Start fertilizing after your average date of last frost (about the 3rd-4th week of May for Aurora).
