Blocking the eye constantly is not the same thing as paint feeding in to the marker ball by ball - it causes an eye error.
When you actually shoot paint, you pull the trigger, the eye verifies that there is a ball present, and so the marker fires. After the shot, the bolt opens, and another ball feeds. For a brief period between the bolt opening and the next ball feeding, the breech is actually empty. The eye registers this, and that is when the board then starts waiting for the eye to see the next ball enter the breech.
If the eye is constantly blocked, the board doesn't know when a ball has fed and when the marker is ready to fire again; from the boards point of view, blocking the eye constantly is actually more like having paint on the eye, then having a ball feed after each shot - hence the eye error.
You may find that your eye will work normally once you have some air and actually shoot some paint.