Having great difficulty getting bluetooth keyboard to connect to my MacBook Pro

After two years without any problem, I am suddenly having a lot of trouble getting my external bluetooth keyboard to connect to my MacBook Pro. (Using separate keyboard because native keyboard has many keys that don't work.)

It's been bad enough (twice so far) that the last time I was about to give up after a half hour of trying to connect when it suddenly finally connected.

How can I determine why this is occurring and, hopefully, fix the problem? 

Although the "Bluetooth Setup Assistant" is supposed to open automatically if a connection isn't detected, that doesn't happen and I don't know where to find it. Information displayed when the 'Advanced' button on the Devices screen (Bluetooth Preferences) is clicked:

"If you use a keyboard and your computer doesn't detect one when you start your computer, the Bluetooth Setup Assistant will open to oonnect the Bluetooth keyboard."

Would there be any point in removing the keyboard in the bluetooth 'devices' list and going through the setup process again? 

Thanks.

Mac OS 10.15.7 (Catalina)   

Original Apple Wireless Keyboard (used less than two years, was new when I started using it).

The keyboard's batteries test in the 'green' -- i.e., as 'good'.