I know, I am really sick of doing the whole thing, over and over, with the secret word trying to figure out what it says, typing it in, and then there's that stupid "there are no tickets available." Save us all the time, trouble, and frustration and say that on the first screen. No, they couldn't possibly do that.
And yes, there is no pattern, which is most likely on purpose, to when you might see tickets. One tour I went to, they had been putting up the best tickets within 1-2 wks before the show at 9-10 am, no set day of the week, but then for the Pond where I wanted to go, they put up the best seats at 9 pm on a Sat. night when the artist was performing at Staples, so most of us wanted to go to the Pond were at Staples.
I won't give up hope, and I'll keep typing it in, until they cut off TM, which they do for most concerts about 2-3 pm day of the show, then it's box office only.
At one show I went to in NC, the mgr of the arena let us in and took us on a tour of the stage setup and how they do it and such, and he said one reason there are tickets on sale the day of show, besides all the other reasons we all know already, is that they often find there are places to put more seats once the stage is set up, and then they sell them. I would expect maybe more GA's as well after U2's stage is set up at each venue. Each arena has a different amount of space of course, and sometimes the stage takes up more, or less space.