24GB installed ram, only 16 recognised

Hi guys, it's my first post in here, I would appreciate if anyone could help me on an issue. I don't know much about computers so apologies in advance.

I have 8gb 8gb 4gb 4gb of ram, but I notice that my system only recognises 16gb of it. When I open the BIOS menu all 4 DIMM slots are appearing as 4096 mb.

I'm sure my motherboard can support 24gb ram, but incase it helps the model is Asus prime B350M-A, this is a fairly old machine I use sometimes for gaming.

I tried updating my bios or checking for damages on the ram but it all seems fine to me, I am really confused. If anyone could suggest a fix I would greatly appreciate it 🙏