Everyone signing up before the battle begins has an equal chance of getting in. People signing up after the battle begins are lined up in a regular queue. Those who signed before the battle, but didn't get in are put in the front of this queue.
Since a few months i have the same problem. I play with 2 accounts, characters on my 1st account only get in after about 00.30 when less then 5 people, characters on my 2nd account always get in. Also when i play on acc no1 and my friends also join, they get in and i don't, 2nd acc np. I always join the queue at 15 mins, so it's not cause i join to late. Made a ticket about it and GM gave me the reaction that "it's just dumb luck who gets in". Said there's nothing you can do. Seems really unfair to me that some people get the chance for free honor and others just don't, but yeh nothing you can do.
There are two pools for TB. One is a pool of people who queued before the battle starts. Another pool is of people who queued during the battle. The game first picks random players from the first pool until there are none left. It then starts picking from the second pool.