Firstly, make sure it isn't a pain issue.
If pain is ruled out by professionals, then I suggest the "Ask, Tell, Demand" method to get a horse to listen to a softer cue.
Phase one is "Ask:" You "ask" the horse nicely to move out. This means a squeeze of the calves, maybe a kiss or verbal command.
If after a few seconds the horse doesn't respond, move to the "Tell" phase: Give the horse a bump with the leg, get more aggressive with your voice.
If the horse still doesn't respond (fairly quickly - don't sit there for 2 minutes in the "Tell" phase) move on to the "Demand" phase: bump with a spur, smack with a whip, growl, etc.. getting increasingly more demanding until you get a response forwards, and cease immediately. Be sure, as Kevin said, not to catch the horse in the mouth, otherwise you'll have undone everything that you just worked on.