Authorising individual tracks

since the start of July when I have downloaded albums not all of the tracks will play. Some do as normal but others require me to enter my Apple ID and password to authorise the computer but then they still don’t play. Not had this problem til last couple of weeks. I tried to create a playlist and burn to a CD and only half the tracks copied and when I explored it the tracks it wouldn’t play hadn’t copied. I have tried signing out and in again, de-authorising and re-authorising the computer, updating to the latest version of iTunes and it’s still the same problem. It is so frustrating! How do I solve this so I can access the music I have paid for and downloaded?