I always look at simplest solutions first. is the plant getting more or less water?. soil diferences? The holes in leaves could also be from high wind in area. That said. If you cannot find the insects on the leaves then a insectice spray may not help ... at this time of year insect damage is not a serious problem.... The brown tips could be water or wind problems or a more common pproblem with this shrub Stem Blight(which is fungal in nature) found in areas where soil is to wet, check stem & see if turning brown or black if so the you do have a problem most easly solved by destroying plant & trying something else.

Many plants around here are showing signs of scorch, brown leaf edges caused by the plants inability to transpire enough moisture fast enough to keep the leaf cool during the hot, dry weather we have had. I would suspect you have the same problem, hot, dry weather with insufficient moisture levels to keep plants healthy. Dogwoods native habitat is swampy areas.
If when these were planted the root balls were not "broken" equally that would explain the difference in growth pattern, the roots of the smaller plant have not moved out in search of moisture and nutrients, unless these were bare root plants.