It's called a ticket drop. Lots of times they trickle out the tickets in blocks. Sometimes they have others that become available again if someone dumps them, or their credit card is rejected, or they go over the allotted limit, lots of reasons. That's why you never give up. I sometimes get them a few hrs later. That's why nothing is ever really "sold out", they hold some back after the public sales for corporate, band members, etc. and usually a day or 2 before, or the day of the show they will release some more.