I have called in several deer and the only time I have scared them off (that I know of) is one or two occassions:

1) I call to loud to make sure they heard it (they did)
2) they had already come in and decided they need to leave for one reason or another, in other words they were to close.

The most visible one I called in was with "the can", he was passing by around 75 yards out (bowhunting). Everytime I flipped the can over, he changed direction to come to me. When he changed it to go away, I flipped it again and he came more towards me. Shot him at 10 yards.

Call soft, they can hear it!

Most often when I can see the deer and call, they either just look over or show no reaction. For it to work you need a deer who actually wants to be called to. Others will just ignore it.