It's a bit late to transplant a tree at 15 feet, and in the first year or so it will be making new roots, and you wil not see much growth. My Raywoods were planted 6 years ago, are now 15/20 feet and gave a spectacular display of colour this fall for the first time.
The Raywood Ash tree is listed as a fast growing tree.
It's yearly growth rate can be about 1 ft per year and will grow faster with more water and fertilizer.
Your tree sounds to be healthy and likely is just becoming established.
Here are some links with more information.