Spray them all. I can guarantee that if you have infected trees on your property all the trees on your property are infested. Look closley on the lower branches toward the inside and underside. Also, I find that the worst accunulation is on the Horth side of my East to West row of mature Hemlocks. The suggested time to spray is mid-March through April. They go into dormancy in July so while spraying at any time except when they are dormany will kill them but spraying when the trees are close to dormant is best for the trees. I spray in the Spring and Fall. It is something that simply is impossible to "cure" and only continuous spraying will save the trees. It is tough to "see" if the spraying is effective as the wax casing (white eggs) remains even after effective spraying.

You sprayed while they were active so you did kill most of them. The oil smothers them and they cannot escape that if you got under and into the center of the trees. The only mistake you have made is not spraying the young trees. They are 100% guaranteed infected.