The courses are staggered, several months apart. The limitation is simulator access. One course must be finishing as a new course enters the sim after 6 weeks of theory. Also, there is a limit to the number of trainees that the centres can accept - the OJTIs (on-the-job-training-instructors) are worn out from training and keeping up with all the changes. If you visit a centre and feel that the controllers are a bit off-hand, don't take it personally.
There is the possibility of maximising simulator access by running courses off-set, morning and afternoon, but unfortunately the restriction at the moment is instructor numbers. And with Airservices refusing to pay potential instructors the equivalent of field salaries, it ain't going to be fixed soon.
After your interview, you're placed on an order of merit. So you may miss out on the next course but be picked up for the following or a later course.
I don't know what Airservices has done with recruiting - it used to be a 6 month process.