Time Machine backup keeps failing

I have a brand new MacBook Pro and the Time Machine backups keep failing, I get error pop-ups saying failure, but doesn't explain why. I've tried 3 different external had drives, the first was the drive from my previous Mac, than 2 brand new hard drives (WD My Passport Ultras). I erased and partitioned them both to Mac OS Extended (Journaled). They worked on the initial Time Machine backup but fail after that. Sometimes it will work after a re-boot, but not always. My setting is set to automatically backup, but it consistently fails. Any help?