Firstly check the halti you have is not too big for him, some cockers need a size one some a size two, unfortunately halti's like any training aide, some dogs learn to pull on them, you need to get in quickly and turn the dogs head (as per instructions), but this is as the dogs head goes just past your leg, don't wait until he is 3 inches or more in front of you, as you will not be able to turn it, he will be at the wrong angle.
Once a dog has learnt to pull on a head collar, it is hard work to stop them, but it can be done, good luck.