Our business website has been working fine but it has started to allow overbooking even though the capacity is always the same. Any ideas what might be going on?
Are you using the Wix Bookings app with or without Wix Corvid?
If you are using the Wix Bookings app without any code, then as this forum is for code related issues only, then you should be going through Wix Support.
https://support.wix.com/en/article/contacting-wix-support
If you are using Wix Bookings with Wix Corvid, then please state what tutorial you have used if any, plus paste up any code in a code block that you have used and any pics if you can.