The typical response from bulls now is to quietly move in the opposite direction. Use calling sparingly and vary the sounds you make to include calf and cow sounds. Much of what it takes to be successful at calling is to learn the right time and place to use this technique. Avoid calling from roads and areas that get a lot of hunting pressure. Avoid calling during the middle of the day.
Patience is very important.
A suspicious bull in not likely to come charging in. A bull may approach quietly and slowly. Calling can often be used to confuse animals when you make noise that spooks them. A call at the strategic time can make the elk think that the noise they just heard and started to spook from is just another elk. Confidence is also very important.