Accounts department sucks!

I paid my child's registration for 2020 on the 27th of September 2019, paid her fees for 2020 on the 13th and 14th of December 2019 and her stationery End of January 2020. I have received calls that i owe the school. How s that possible? I even sent them a proof of payment for all the above but they only entered 1. Their accounts department is such a nuisance. The worst experience ever.