My first thought is your tree is possibly planted too deep and the rootflare is buried and/or there is too much mulch.* Here's some handy sites for you to check to see if that is the problem.
http://www.mygardenguide.com/care/Ro...0disorders.pdf
http://www.tlcfortrees.info/planting%20depth.htm
http://hort.ifas.ufl.edu/woody/overmulched.html
http://www.ext.nodak.edu/extpubs/pla...ees/f1147w.htm
http://www.treesaregood.com/treecare/mulching.aspx
My second thought is root damage during transplant or the tree was so rootbound that it can't take up water in the center of the rootball.
http://hort.ifas.ufl.edu/woody/rootg...ontainers.html
Here's a great site about Japanese maples.* Do read the Articles, FAQ's and How To.
http://www.mountainmaples.com/WS4D_C..._15/index.html
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.