Hmmnn..if the chuck turns a little off the shaft it should finish coming off..is the screw completely removed?..also make sure you are all the way in low gear..you may be in neutral?..you may also try hitting the allen wrench left handed..hee..hee. just kidding..I presume the drill is assembled as you are trying this..the more resistance you have on the shaft the easier it is to remove the chuck..remember it has to be a really sharp blow on the wrench to knock the chuck loose...
Craig