Downloaded songs on phone but wont download on computer

Hi

I tend to download songs via the iphone. I have now jumped on my computer to sort my music and while some songs had downloaded, others are saying they are are not authorised to use on my computer. I have deactivated my computer and reactivated it, which hasn't worked. I have checked the download list on my ipad and iphone are they are under purchases. I don't know what else to do????? Why does it allow some to download but not others?? I begrudge having to pay for them again.