I have a Brass Lion door Knocker and a Clearly indicated White bell push and have the same problem. The Express Delivery drivers do use the Knocker though.
Timid knocks on the wooden panel of the door are the normal. I think they want to get your attention but not really ready for who might answer the door and shoo them away.
It is the reason they are knocking that may give you a clue. Do they want you to buy something or deliver something.
I rang a bell at a neighbours house and they thanked me, saying please come in, we don't have many visitors.