If I knew that it would be so cock up like this, I would've booked when there were available slots that time. Btw, does this mean that there are no more slots available at all?
Ok, info here is quite dated, but this is how it works when it all started.
No more slots during your window, so need to get "deferment" to book RT slot, or just wait for them to book for you and send you SAF100 for it. By right, there is no law to say you must go for RT, or at least not chargeable by summary trial (ie must go court). So they give you a window to book your RT, if you haven book, they will book for you and send you SAF100 (with SAF100 or you book yourself, can charge already if you dun go). Usually all these people need to grant deferment by default as no more slot within their window already by the time the unit books for them and sends the SAF100.
Also, RT slots also get taken up in blocks, like when people get called back for charges for defaulting IPPT, they are required to book their RT right on the spot, so if that day got 20 to 30 people kena charged for defaulting IPPT, 20 - 30 RT slots will get taken up in a short time, not to mention those booking at home by themselves. Similarly RT slots get taken up when the unit books for NSmen and send them SAF100, I cant remember the exact statistics as to how many we book at one time, but can be up to a hundred at a time, so if you happen to book after that, then sure a few slots taken.
System isn't cock-up, everything is subject to availability (aka while stocks last), not system's fault if you refuse to book early. Anyway, you can change your RT slot as long as there is availability.