The chance is high that you bought a false ticket or a ticket that was resold several times or a ticket that is part of an official investigation against the (re)seller. It is therefore best to file a complaint.
Websites where you can complain against fraudulent ticket (re)sellers:
If you paid for your tickets with a debit or credit card, you can also file a complaint through that channel (mijnkaart.be, your bank, ATOS, Amex...) and claim your money back. If you confront the retailer himself, he will often demand that you provide proof that you did not get in. Be•at Tickets staff at the box office can confirm that you were refused entry. This may help you when you want to claim your money back from the retailer or when you make a complaint through another channel.
NOTE: Many resellers work from abroad (even if they call themselves ‘ticketsBelgium’) and therefore escape the net of the Belgian legislator.