Feathers are forgiving, but they are not supposed to hit the shelf each time you shoot, or they will show premature wearing. The contact is likely caused by one or both of the following problems.
1) As StevenB suggests, your string nock may be too low. The arrow is launched too close to your rest or shelf and makes contact.
2) Your arrow is too stiff. It does not bend enough to give adequate clearance as archer's paradox allows the shaft to pass around the riser and causes the lower hen feather to strike the shelf.
Shooting cock feather in, as J-san describes, is a method to cope with a marginally low nock point or stiff shaft.
At some point you will want to bareshaft tune your bow to make sure your nock point is set correctly and your arrow's spine is matched to your bow. There is more than one way of doing this. I think the
method on Morrison Archery is fairly simple.