I had the same issue. I got every W-2, a paycheck stub from every job, a boss contact from every job, all my school transcripts, and pretty much any and all paper work I could come up with and uploaded to the background check ppl.
Then I bugged the background check people literally every 2 to 3 days and asked the status.
I would push back your start date until it's confirmed though to be honest.
I don't even have a contact with the background people, I filled out information on the third party site, and they finished report within a day or two. But they couldn't reach the companies, and didn't ask me about any proof, just wrote a report that they couldn't reach them.
Then I bugged the background check people literally every 2 to 3 days and asked the status.
I would push back your start date until it's confirmed though to be honest.