When you say you have checked your library, what was the file path to that library?
Have you checked the options in the brush panel? I like to use the right click panel (with brush tool selected, right click anywhere in the document window). Click on the little cog icon in the top right corner, and the groups not in the main panel area should be available there. When you delete a group from the main panel, they are still available from the library.
Note: I have just noticed that my groups are doubled up and I don't know why.

The other thing to check is previous versions. The file path below is for Windows 10
All Photoshop file locations can be found here
C:\Users\[your user name]\AppData\Roaming\Adobe
As you can see, there are all the previous version that were installed on your system since the last OS install. When I installed the current version, I had to drag the brush groups from the previous version Presets folder, to the current one.

Good luck, and let us know how you get on.