If you are expecting payment from your customer for an order made with a purchase order and have extended Net 30 terms, contact the customer to find out why you are not being paid. Most times, it is a bureaucratic "red tape" problem within the organization. Simply track down the responsible party in the organization and continue to follow-up with them until payment is received. It's important to be persistent and business-like when working to obtain your money for net term payments–as the longer you wait, the more difficult it can become to get your money.
Bad checks or NSF checks are often considered criminal offenses and can be taken care of through small claims court.
Contact your local district judge or local authorities to see how to handle this. In the instances when it is obvious that you are being given "the run-around" and not going to be paid, you will have to take stronger measures. For information on how to deal with customers who just will not pay, check out the article on "Dealing With Deadbeats."
You can not remain in business if you do not get paid. After all, it has already cost you money because you bought the shirts from your blanks vendor and printed them. And you certainly do not want to be stuck with a box of shirts with someone else's company name or message.