Can't change app icons on M4 MacBook Pro

I had an M1 MBP and had always customised many app icons (not the stock apps, third party apps). I've upgraded to a brand new M4 MBP and can't change any of the icons as I had previously - I copy an icon to my clipboard, find the app, go to Get Info, select the icon in the top of the Get Info window, paste, but nothing! I still have my M1 MBP here and tested it on that and it works fine! Can anyone help? Only a minor thing but it's bugging me why it works on M1 MBP and not on a brand new M4 MBP.