Unpaid medical aid scheme contribution

I have been a member of GEMS since 2010 .I always had a good relationship with scheme until last year 2020 in October when I received a SMS informing me about an unpaid contribution which surprised me because my deductions are made through persal. I was eventually informed by the medical aid scheme that the outstanding amount was for my son when I wrote an e-mail for enquiries. This after I informed the scheme that they should remove my son as a dependent as he was done studying and independent early March 1919. They say the amount came about because they did not deduct the correct amount for his last contribution. I spent the festive season sick and unable to go to the doctor as I was and still suspended for that amount when I wanted to know why I was not informed properly about this situation but my monthly contribution is collected without fail.I am even harassed every day through messages and e-mails. Why did the scheme take such a long time to inform me about this matter.