The original Loop de Loop had 2 problems that resulted in about a 10% fail rate for the loop de loop:

• The drop was too sharp at the top causing balls to get a little air time, and
• The exit from the loop was too small and too tight, causing some balls to jump the track on exit.

I am now getting 100% success rate with this new set (after some tweaking, I haven't had a ball jump after a few hundred runs =D ).

https://www.youtube.com/watch?v=jpIeFuiiIfE&feature=youtu.be

A few notes:

• I printed the drop on its side
• Both printed with support everywhere and cleaned up after.
• The most important part for success is a smooth entrance into the loop, so try to remove any jostling caused by the connector between the drop and the loop. You may need to shave down or sand parts, depending on how they fit together. If the ball enters the loop smoothly, it will work! This is what took me the most time to tweak.
• Also make sure there isn't any left over little support pieces being hit by the ball.