Some of the Mk2a sleds didn't have their top lips folded up. While I've never seen this be a problem in practice (it seems to fit in a slot in the back of the player's fascia just above the lens), it might be a problem for you.

See if you have the (slightly) defective sled, then see if it might actually be affecting the player's ability to completely enter the sled.

More information about this is available somewhere in the FAQ.