License Key won't work

I have been a licensed user of Advanced Renamer since 2017. I have just done a clean install of Windows 10, having previously used Windows 7. When I re-install Advanced Renamer and enter my license key, I am told that it is an invalid key. What is going on? Surely I don't have to buy a new license in order to re-install? I have tried it with both the lastest version of Advanced Renamer and the old 2017 version which I saved when I first registered the software.

Reply to #1: Ok, I've just restarted my computer and tried it again (fifth time lucky) and this time it has worked. That's a relief!