I have just added a new work item type using the witadmin tool:
witadmin.exe importwitd /collection:\"http://mytfsserver.com:8080/tfs/myprojcol\" /p:\"myproj\" /
It is a caching problem.
Try running:
witadmin.exe rebuildcache /collection:"http://mytfsserver:8080/tfs/myprojcol"
Afterwards you should see your recently added WIT.