Originally Posted by Hardyreels
Water load the rod and use a single back cast. Allow the back cast to straiten out and then let it fly. Practice with positioning the line prior to the "water load" will help you to make the presentation cast where you want.
This is good advice and it sounds like it may be your problem particularly on the back cast. Large heavy flies need to be allowed to go as far back on the back cast as you can get them without falling off at the end of the back cast to be able to shoot forward with the greatest velocity. If your back cast timing is off by not waiting long enough before you forward cast, the weight of the fly will work against you when you cast forward by slowing down before the weight can pull the line forward shooting the line. If your timing is off your forward cast will tend to collapse before it has a chance to shoot the line. Try waiting a little longer on the back cast before you start your forward cast.