Sounds like a weird issue. You could have malware trying to remove administrative privileges from all users. I would suggest running Malwarebytes to see if it finds anything more-so as a precaution.
Try these steps as well (after the scan):
1. Click Start.
2. Click Computer (you can also found this icon on the desktop).
3. Right click on the Hard Disk icon where your OS is installed on and click Properties.
4. Click the Security tab.
5. Click the Advanced tab.
6. Click the Change Permissions button located after the Permission Entries list.
7. A new window will appear on your screen. Such a window contains a list of all the user accounts available on your computer.
8. Select the user account you want to give total control over your Windows 7 and click the Edit button.
9. Now, tick the checkbox labeled “Total Control” and press OK.