What's taking my employer so long to contact me after my drug test?

I've been hired by a mall in Texas as a security guard as of May 28th. I took my drug test the same day. The official job offer contract stated I'd start before or on June 6th. They told me after the drug test that they'd call me back for the next steps. It's June 6th now and I haven't heard back at all. I've called 3 times now and each time I call, the hiring manager isn't there.

I heard it takes some time for a back ground check to get done on security guards but surely it shouldn't take this long, especially when we're passing the date of start on the contract!

What could be going on? Should I call back again? What would you do in a situation like this?

Thanks!