Q: Switching iTunes from PC to Mac

I have just switched my system from a pc to the iMac.  Have two iPhones and two iPads synced to the account, along with Apple TV.  However, it seems that there are now two different iTunes accounts/libraries.  Several of the items that were purchased prior to the switch no longer appear in the new account and new purchases on the iMac don't show up on my other devices such as iPad.  Any suggestions to fix and/or how I incorrectly set this up.