How can I make sure that all songs sync to my iPod? I have a 160GB iPod and it only syncs old music to my iPod. 

I buy new songs and it will not sync them. I have probably 150 songs that will not sync (purchased songs). I did run diagnostic and it says iTunes helper is not working. Not sure what that is and not sure if it has anything to do with it.

How can I fix so that all songs sync to my iPod?
Make sure that in iTunes that you have it set to sync your "Entire Music Library":


Open iTunes*
Connect your iPod
Select it in the iTunes sidebar
Click the "Music" tab
Ensure "Sync Music" is checked
Select "Entire Music Library"


*This assumes you are running a fairly recent version of iTunes.