Hi Geo,
There are many possibilities as to why it's not growing so much... too much water, too little water, the amendments in the hole, nematodes, etc. Probably the best reason is the tree itself. I have several trees I planted (along with many rooted cutting) this year which have put on tremendous growth. I have three LSU Purple and three Black Mission trees (each from the same parent) which under the same conditions as the rest have put on little growth. It's a bit late in the season for fertilizer. I'd wait to see next year's growth before worrying.
Interestingly enough, one of the LSU Purple in the group was completely defoliated by grasshoppers, it came back and grew to the same size as the others and stopped.
~james