You’ve got the full explanation as to what has happened on this thread:
Enhanced newbie restrictions & requirements. On September 17th 2018, all Jr. Members who had not earned any merits at the time were demoted to Newbies. That also came along with the restriction for Newbies from bearing a signature, although a
copper membership can bypass this restriction.
If rank is of importance to you, you need to start posting with much more effort than what you’ve put in so far, enhancing the content of your posts. Stay away from spam megathreads as @BitCryptex suggests, and specifically focus on giving more depth to your posts at the expense of time. Remember that there are many posts per day that compete to some extent for merit, so you need to shine a bit in order to spotted in the first place.