Answer: When plants fail to grow quickly, it is sometimes due to a failure to root out beyond the original potting soil or root ball. But since your plants seem to be blooming well, they are probably healthy enough and I would not expect this to be the problem.
This variety has been reported as a bit slower growing when compared to the species and there is also, as one might expect, somewhat of a variation on mature size depending on climate and growing conditions. (Most references I am finding on Peppermint Lace list it in the mid range of 8 to 12 feet at maturity.) On the other hand, they should have grown faster than what you have described.
These plants will grow best in full sun all day long so reduced light can slow them down. Make sure the soil is of average or better fertility, and that they deeply watered in times of drought. Water is probably the most important limiting factor to growth overall, and although drought tolerant, crepe myrtles will respond well to adequate moisture throughout the growing season.
Your county extension should be able to help you with some basic soil tests and interpreting the results to make sure any fertilization needs are being met. They may also be able to help you trouble shoot for any cultural problems that could be slowing them down.
Finally, there is always an outside chance that the plants were mismarked and you have an overall smaller variety. Comparing the blooms to plants you know are the real thing would be the best way to double check that possibility. The reason I mention this is that I am unable to find a reference to any Crepe Myrtle called Cinnamon Ruffle, although the Peppermint Lace is easily located.
I hope this helps you trouble shoot.
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