I have had to open itunes on a new computer because my last one died, and I didn't have any backup. Most of my purchased music I got back, but some songs didn't appear and i have to buy them again. Why??

I've had to open itunes on a new computer as my last one died and i didn't have backup. but i've noticed many of my purchased songs have not come back to my itunes to download, which means i have to buy them again. Can anyone help me? Most of my purchased music was there ready to load, but about ten weren't. thanks.