The Write Automation is having no effect in mixing. As I have done many times, when I am mixing a given bus, having activated the "W" (glowing red), and having slid the fader in a given section to remix my bus, there is no change in playback...the levels stay the same as before. I have rebooted, but didn't help. Thanks.

Did you have the edit filter set to automation and volume?

be sure read automation is active

I don't see anyplace in a bus to set an Edit Filter. Doesn't that just apply to tracks? I have several tracks feeding into one bus, and I* want to change the general level of all those tracks. Do I have to go back and choose Volume in each track? Have never had to before - unless I didn't realize that all happened to be set to Volume...but I don't think so.

What is Read Automation? The glowing "W"? As I said, it was active.

The "R" should be blue it is located next to the "W"

I agree that it sounds like automation read is off.  Is the automation info being written?  Can you see the envelope?
 
Are you using a controller or your mouse to move the fader?
 
Are you playing the track when you move the fader?
 
And yes, the busses have an edit filter too. 

The Read (R - white or gray, not blue) is activated, I use the mouse to move the sliders while playing the track. I found the track Edit Filter in Clips>Automation>Volume, but do not see where this would be while in the Bus window. Once again, I have been able to mix the same way I'm trying now with no problem, and as far as I know, I have not changed any settings - it just quit responding.

Could it be Offset mode? Do you see a "+" somewhere? IIRC you toggle it with letter  "o".

The only "+"s are for Effects and Send.