That's what is happening to you. You are letting the speed get too slow in the top of the loop or you are trying to loop too tightly for your plane and power combination. One wing stalls a little before the other and it drops. If you hold the controls the plane will go all the way around in what is called a snap roll.

If you have dual rates on your plane, try looping on low rate and see what happens. If you have the power, you should get a larger loop. If you are low on power you may fall out of the top, in which case, dial your low rate up some and try again.

The Dragon Lady 40 should be a great plane to learn arco, especially if you have a .46 for power.

It is funny you should mention a loop with a snap as I write for R/C Report magazine, covering one or two aerobatic maneuvers every month in my column, Fun Aerobatics. Today, I sent in the June column which was loops with snaps at the top!

If I can be of any more help, please e-mail.