Answer: In any batch of plants some will be less vigorous than others. The reasons vary: some of the plants were probably less well developed when planted; the soil may be different; perhaps some received less water than the others; neighboring plants were extra vigorous and received more light or nutrients. Some plants simply don't adjust as well to transplanting. The difference is probably due to a combination of factors. Eventually, perhaps in just a year or two, you should not be able to tell the difference. Once they have been clipped regularly they will blend together.
It's a good idea to begin training a hedge early so it develops a dense framework at the base. Initially this means trimming it off fairly short to encourage branching low to the ground. I know, this is so hard to do when you want a hedge as quickly as possible, but it is important. You also need to trim it often to keep that framework branching. This is what makes a hedge dense. You also need to trim it so that the sides are sloped. The hedge should be wider at the base and slightly narrower at the top so that sunlight can reach the lower branches. Shaded branches won't grow as well and will end up with sparse leaves. The best times to trim are in early spring and through the summer into early fall. If you prune now, it may stimulate new twig growth, which would break the hardiness the photinia has developed, leaving it succeptible to cold damage.
Finally, it does take time to grow a good hedge, even with a fast grower like photinia. Good luck with your hedge!
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