Yes its tough to get a seat everywhere, just sign up for what you can get. They reduced the number of seats per exam this year so that made it even tougher to get in. I hear that some changes may be in the works as far as test format, that might make it easier to grade and therefore let more folks take the test. Right now two National judges have to grade each test, very time consuming and labor intensive.
You might also let places know if you can fill in at the lat minute, I had a couple of chances to retake my tasting portion recently if I was willing to go the next week. I didn't take them up on it but only bacause I passed the test already and am not in a huge hurry to get to the next level.
In the meantime, sign up to do some judging as an apprentice, or better yet steward. It's great experience and will probably help you as much as studying. My wife has stewarded a couple of times when I was judging, and honestly I think it might be more fun to steward. You can ask to taste the beers and you hear judges discussing so you're really a part of the process.