You need to turn on DEBUG logging for org.jboss.cache.TreeCache and org.jboss.cache.statetransfer on both the newly joining node and the coordinator. The logs should give you some idea what's going on. It's most likely either:
1) There was some problem preparing the state for transfer; you would see errors on the coordinator.
2) Your InitialStateRetrievalTimeout is too low -- the request is timing out before the state is returned.
3) There is some communication problem between the nodes.