We've had them howl at us and not come in. Usually when we have them howl at us is when we started our stand by howling. If they did the warning bark, then yes, you were busted before you started. A warning bark is a series of short sharp barks followed by short sharp howls, over and over.
Sometimes, no matter what you do, they won't come to the call. Just make sure you get into the stand without them knowing that you are there. Don't let them see you get there and don't let them smell you. If they know you are there, they simply aren't going to come.
If you think they are within 200 meters of you, go to the lip squeeks, or the "kiss of death". They will be able to hear that from 200 meters easily, unless it's very windy. The lip squeek is an excellent sound to bring them on in.