For anyone else dealing with this issue, there are three common issues that can cause your license not to stick.
1.) Wrong PC ID. Newer builds of Mach4 will tell you when your PC ID is wrong, so that's easier to diagnose. Your PC ID is shown in the Help>About window in Mach4 and is used to generate your license. If it's wrong, you can fix it by logging in at machsupport.com, going to 'Manage my licenses,' and entering the correct PC ID before clicking 'Add New.'
2.) Not running Mach4 as an administrator. If you're not sure, best to double check by right-clicking the shortcut and selecting 'Run as Administrator.' Windows might ask you for your password. That's your password and not anything I can help you with (sorry!). To make this a permanent selection, right-click the shortcut and go to Properties, then click "Advanced" in the shortcut tab. Select the radio button that says 'Run as Administrator.' This just saves you from having to right-click every time.
3.) Antivirus interference. If you're running an antivirus program on your computer while trying to use Mach4, you're asking for trouble. I recommend not running any programs at the same time as Mach4 (especially when cutting a part!), but AV programs especially can be an annoyance when trying to do things like license your software. You can try adding it to your whitelist (instructions on that are dependent on which AV program you use) but it's safer to just turn it off while using Mach4. Windows Defender, at least on Windows10, doesn't seem to cause this issue. I leave that running on my computer and haven't had an issue the dozens of times I've licensed and relicensed new builds.
Licensing issues are support issues and you can always reach out to the team at support.machsupport.com. Avoid using the Contact Us form for licensing/support because that sends your inquiries to an email rather than the ticketing system that we can all access.
Happy CNCing!