I get credits back *every month* as an advertiser.
But email support and ask them. I had a larger than usual drop once (it actually was just around ten bucks or so, but that was larger than usual) and I emailed and it turned out that the advertiser was doing something wrong so that's why I didn't get paid.
Did you have a spike of traffic from an unusual source? Possibly an advertiser or advertisers defaulted on payment, or got hacked (they don't usually have to pay for that, which means you don't get paid for it)