Horses that don't 'go' either had muddy training or too low/onto the forehand/out of balance. So the rider's job is to EDUCATE THE HORSE to the use of the leg. That starts ON THE GROUND. Work the horse in hand, touch the horse where the leg would be (with the hand/with the whip); that touch is PROGRESSIVE. Touching (with the whip) is touch/vibrate/TWACK. Very rarely does the rider have to do that more than a few times and the horse WILL learn.
With a green(er) horse, the noise of a bat on the shoulder WILL send the horse forward, and which point the rider combines that with a touch of the leg. The horse MUST be allowed to come up/open/active (NO 'shaping the horse'/making it rounder yet. For an older horse the whip is used behind the CALF (NOT not on the butt) and the rider must NOT get left behind/grab the mouth/stop the horses reaction. Again, touch/vibrate/thwack. No horse should take more than two or three times to learn that IF the rider's actions are CLEAR.
The spur is NOT for forward, it is for specific action/nuance.