See if you can lookup any documentation on the policy, such as in their student handbook or similar, and read every line of it. Do as much of it as you can without their official input, whatever it is - fill out a form, serve notice in writing, etc. In short, make it difficult on whatever bureaucrat is dropping the ball to say you didn't make a sufficient good faith attempt at following their policy.
This can include sending whatever your local equivalent of US "certified mail" (provides proof of sending and receipt, requires a signature of acceptance on delivery) to the registrar's office. Some legally recognized equivalent of this will make them much less likely to be able to successfully pretend you did not follow the written policy, and in so doing will make them less likely to even attempt it. This assumes the whole institution isn't a sham, of course, but I assume here they are at least attempting to function properly but are having a very typical bureaucratic failing of this sort - especially as it nears the beginning of a new semester and they are likely very busy.
As I've run into these sorts of issues before, it is also good to loop in your assigned advisor, if you have one (in writing/email at least to leave a paper trail), and possibly also the chair/department head of your major/graduate school program.
Most things in academia end up coming down to needing some department head and/or dean to sign off on it, so if you've served proper notice to the relevant academic departments you will find dealing with the administrators on the institutional/infrastructure side much easier. Most places like to play at least a little bit of ping-pong, sending you back and forth for signatures or the like, so good to cover all your bases. But most of all, it is important to know the policy to the best extent you can, so really put in extra time and effort to find a written policy (online or in person), read it closely, and keep a copy of it for reference.