I only had trouble once clearing customs, but it was a problem with the way the boxes were labeled by the supplier, and not a problem with DHL. Is this a DHL problem or a customs problem?

Depends on your product. DHL or any shipper has no influence here. It’s up to customs. It’s usually just 1 or 2 days but it can take weeks. They may request more information about your product. The FDA may examine your shipment and hold it up longer. I once had a shipment that was rejected by the FDA. It had to be destroyed. I hope that doest happen to you.