Generally speaking, there are two things you can try to have these backups play in cd players:
1) Try writing at lower speed (you already tried the lowest speed)
2) Use better quality media
If the above won't work, it is most likely your player that is not able to read the backups (due to weak laser or any other reason).
Perhaps you can find other solutions and/or discussions by using the search, this is a common problem and discussed before.