Cant sync iPhone because itunes says this computer is no longer authorised

whenever i try to sync my iPhone 5s to my computer (iMac.. the older/thick one), it backs up and works fine until it starts to download music and apps. it says that it cant sync to my iPhone because the computer is no longer authorised for the purchases made on the phone, it gave me the option to authorise (which it already was) so i did, still didn't work, so i reauthorised and reauthorised a couple of times. i first started to notice the problem when i updates to IOS 8 (iTunes v12.0.1.26) so i don't know whats at fault. i have a 4th-gen ipod touch (6.1.2) that syncs without any problems. i dont know what to do anymore 😢, i just want my new songs on my phone. is there any way i can fix this because having to carry my ipod and my phone everywhere is getting quite tedious.