Two Ways to Cast a Straight Line
One thing I teach people to do on a regular basis is how to get the line to land on the water straight - not in a pile that is. When your line lands straight the fly comes under tension and begins fishing right away. It stays off the bottom in slow water. You will catch more fish if your line lands straight.
One of the most common reasons (and easiest to adjust) is using too much power... using the force necessary for an 80 foot cast when you're only casting 60 feet. The line reaches the end and bounces back... not good. Use only as much power as you need for the amount of line you're casting to turn over and land.
Another reason this happens (this one is harder to diagnose yourself) is that when you go to make your casting stroke the rod begins at 11:00 or later rather than 10:00 (you know, 10 to 2). I isolate the circle spey and double spey into three steps. Step one - set the anchor. Step two - lift and load combo. Step three - the forward stroke. The casting mistake I am referring to is very similar to "creep". During the lift and loading sweep (step 2) the mistake happens when the angler sweeps the rod around then pulls it in close, lifting the rod tip to 11:00 or 11:30. The casting stroke is then not long enough to leverage the line to turn over. The solution is to sweep the rod with a lift out and around to 10:00, making sure the rod tip is tilted back slightly so that you get a nice long casting stroke forward to 2:00.
So, if your line is landing in a pile make sure you aren't over powering the forward stroke, and make sure the rod is tilted back to an elevated 10:00 before making the forward stroke.