When Harry steps into the trick step after listening to the egg in the prefect's bathroom he drops the egg and the map down the stairs. He tries to touch the map with his wand, but cannot reach, and the egg has bounced down the stairs. Why not summon them back (accio map, accio egg)? We know that the map can be summoned, Moody (Crouch) summons it in the same scene.

Also, while he is trying to release Ron from the merperson statue in the lake, why not accio knife to retrieve the knife Sirius gave him?