"It would be nice to know exactly what the problem is..."

It's not complicated. People are walking out of the store with items they have not paid for.

Dollar Tree/Family are reporting similar problems:
newsnationnow.com - Dollar tree theft closures

The solution may be more complicated. DG management mention eliminating self-checkout completely. That implies having to hire more employees.

I have been in a few Dollar General stores where there has only been one employee in the store. At one here in rural Colorado I had to hunt down the employee who was stocking shelves and acting as cashier and manager all at the same time. There were a few other customers in the store at the same time. We all had to wait in line up front until the DG employee came free from stocking shelves to check us out. It would have been easy to pick items off the shelves and just walk out without paying.

Costco has employees standing at the exit checking every customer's receipt agains items in their cart. Costco has very low shrink.

"if it is cultural slippage, where stealing is becoming morally and socially acceptable, or trivialized (police refusing to prosecute small crimes)..."

I think you have struck at the root of the problem.