I've found Time machine buggy since day one. Maybe it's all the stuff I choose to protect from its grasp? It seems to work for a few months, then have trouble finding older copies of stuff. I've always used Super Duper. Now I know longer bother with Time machine, unless I'm doing a specific project that could benefit from hourly BU. When done with such, I turn Time Machine off. For me, it's always offered a false sense of security.