I applied to a job months ago, had an interview then a background check, but job offer was thrown out?

I returned today after not hearing anything back for a while, i know the people working there well and i was told i failed my background check... but that’s it. The manager told me they don’t know why i failed it, and I wasn’t called back by HR or the management that conducted the background check.

How do i figure out what went wrong with my background check and fix it? or could i do a background check on myself?

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