Itunes wont put purchased songs on Ipod PLEASE HELP!

Okay, So I have recently gotten a new ipod nano. I had also gotten a brand new computer. So when I uploaded all of my songs from my old ipod to my new computer everything was fine. I very easily put the songs on my new computer onto my new ipod. A few weeks later my computer crashed and I was forced to reset all the settings, which deleted itunes. I have just re-uploaded itunes onto my computer and I got all the songs on my Ipod to transfer to my computer. After that I began purchasing new songs and they wont download to my Ipod.
PLEASE HELP!

Did you uncheck the 'Manually manage music and videos' box?

Also, the option to transfer purchases from the iPod will be unavailable while it's syncing

Can't think of anything else

how do i change that?

When your iPod is connected, simply click on it in iTunes, then on the Summary page (where you can also Check for Update or Restore), the Options are under Version

You may need to Authorize the computer to talk to your new iPod. Open iTunes, click on Store on the menu at the top of the screen and then on Authorize Computer.