Shoot it offhand and/or without the sled. If it doesn't drop out you'll have your answer. Could be the lever hitting your hand during recoil and/or a weak lever latch spring, or the latch itself is worn, etc.
I had to take mine out, ream out the hole slightly (it had a burr), polish the latch face and reassemble. Haven't had a problem since...
Hope this helps...but it's most likely the lever hitting your middle finger during recoil just enough to kick out the lever. Try changing your grip... :grin: