Imagine getting up before daylight, heading to the woods and setting up in a spot where you think there are turkeys.  Then you wait till the sun starts coming up and the woods are coming alive before you start calling and nothing happens.  You know there are turkeys in the area; you have seen their sign several times, so you keep calling.  After several minutes of calling, finally that thunderous boom of a turkey gobble is heard in the quiet of the wilderness, only he is a long way away and leaving.  He is going away from you!  You could get up and try to circle around and get in front of him.  Then try to call him in, but by now he is all henned up, (in pursuit of a female turkey), so it is going to be very difficult to pull this off.  This could have been avoided through proper scouting and roosting a gobbler.  It is way easier in my opinion to kill a gobbler this way.  You cannot shoot them off the roost it is illegal and highly unethical.  To roost a turkey you have to find an area where there are plenty of turkeys. Once you know where they are its pretty simple, all you have to do is slip into the woods and listen for them to fly up into the trees.  The turkey will usually do this about 30 minute before dark and they will normally roost in the same area.   I am not saying they will roost in the same tree every time, but generally within a 100 yd radius.  Gobblers will normally gobble once on the roost.  In the event they have not said anything while on the roost, you can usually get one to gobble by using a shock call, like a hoot tube, crow call, woodpecker, screaming  hawk or even a peacock call sound.  Yes, a peacock call sound!  I really don't know why, but this will make a male turkey gobble when the best turkey calls won't.  You will not get him to come to you using the shock calls, but you can make them gobble.  Once you have a turkey roosted you can come back the next morning before daylight and set up.   Start calling softly once the woods start waking up.  Most of the time he will fly down and come right to you; the rest is up to you.  

Now go forth and hunt!