Why can't I transfer a certain license to a new domain?

When you've received a license issue, but it does not offer you to transfer the license (instead only offering the option to assign a license from your account, or telling you to buy a new license), that typically means a problem related to the previously licensed domain.

If the license page does not tell you what the previous domain was, the license check was unable of detecting that automatically. That can happen when a license check was triggered by checking for updates in the package manager, rather than using the extra in the manager (e.g. editing a resource or viewing its component). In that case, you may still be able of connecting the old license by triggering an additional license check from the site's manager. Instructions for that will be shown on the invalid license page. 

If the license page does tell you the previous domain, but still doesn't offer you the option to transfer the license, that means the previous domain was covered by a free development license, but the new domain is not. You will need to assign a paid-for license in this case.