Attempting To DFU Devices

Hi - can anyone help shed some light on an issue I have please whereby whenever I try to DFU devices, for example an iPad 7th gen, I put the device in to DFU mode all OK, my iMac recognises that a device in DFU mode is attached, then the prompt comes up for me to restore/update & restore. Once I hit the button to do so, I get a box stating "Downloading 1 item" with "Downloading..." underneath it, then a grey bar and nothing else.

The download progress never begins (or ends) and it just remains stuck like that. The devices consequently come out of DFU mode after 15 minutes. Repeat the process and the same happens again.

I am currently running macOS Catalina V.10.15.7

Any ideas guys?

Thanks.