Open Question: how to download stuff on itunes i already bought.?

i bought a new iMac and have authorized the itunes account. i have a mac laptop that i already bought stuff via itunes on already with my account. when i go to the imac and click on "check for available download" its suppose to download everything i have bought via that account but it does nothing. so when i go to buy something i already bought individually it says that my account will be charged again... why is this?