I am not sure but I may know what's happening. Are these accounts that you're trying to access are old accounts, say more than 6 months old? Youtube users had to connect their accounts to Google ones for extra security and we had to create a new password. In case you didn't do this for your old accounts, it may explain why you can't access them now. There was a limited amount of time given to users to comply with that new policy.
I hope my answer has helped you and that your problem will be resolved.
