Hi Phil,
Please try the options below to check the outcome:
Option1. In the old admin center, click SERVICE SETTINGS>Updates. Check if it is selected as First Release>Entire organization. If yes, select Standard, after a while, switch back to Entire organization. If not, select Entire organization to
see the result.
Note: Please firstly sign out your account and then re-sign in after re-enabling First Release.
Option2. In the First Release enabled situation, check if you can access the preview admin center via this URL
https://portal.office.com/AdminPortal/Home?switchtomoderndefault=true#/homepage directly.
Option3.In the First Release enabled situation, can you see the screenshot below? If yes, click the red section to see if you can switch to the preview admin center.
Moreover, you may change to another browser for accessing Office 365 admin center. Check you will be redirected to the old or preview admin center.
Let us know the results when you have time.
Best regards,
Tim