Time machine back up not working

I have a 2013 iMac running on Catalina 10.15.7. It kept telling me that the back up was failing so I bought another external drive. I've followed the instructions to ensure it is formatted correctly etc but now it just keeps saying "preparing back up" but never backs up. I've backed it up manually for now but how do I get it to back up via Time Machine again? I believe a 2013 iMac cannot be further updated, I've tried two different external drives, I have formatted the new one as per the instructions and it seems to be correct. I can't think of anything else! Has anyone had a similar experience and managed to resolve it? TIA