“Red Tip” plant
We have three "Red Tips" in our garden here in Coastal South Carolina - two are growing "UP" nicely (now about 8'), the third is apparently happy and healthy, but only about 5' high. All three are in the same area, planted at the same time, same sunlight, same moisture and the three are separated by two small Magnolias. Since they are a privacy screen between backyards, we would prefer a bit more height, but the middle rascal isn't cooperating. Any thoughts?
You may want to do a soil test, though the plants are all generally planted in the same location it is certainly possible for the soil conditions to differ from plant to plant.
Here are some links that should help you.