Found out the problem on my own. All is well now. I have 2 other YN 600's and the FEC w/bounce worked fine on those other two. So I kept tinkering with my 3rd/new flash until I got it working properly. Certainly a case of "user error" here.
Here's the story: I have an event coming up soon where I expect to be using bounce flash "on the move." It's been a while since I've done "event" photography, so I wanted to be well-prepared.
I was going over all my gear, settings, methods I wanted to use, etc. That's when I was practicing bounce flash with my new YN600 and FEC was not working. When I slowed down and went thru all my steps with each flash very slowly, I found that it was not a flash problem at all - the photographer was the problem!
On my Canon 6D, I have the "Set" button on the back assigned to be my FEC button. I can access the FEC control by just hitting the SET button. But after you set the FEC you want, you need to hit the SET button again in order for the change to register and return to shooting mode. Problem was that I was being inconsistent in how I set FEC after making the change I wanted. Sometimes I was hitting the SET button to return to shooting mode (correct), and sometimes I was soft-pressing the SHUTTER button (this did nothing - you return to normal shooting mode, but no change is implemented). I simply was not paying attention to what I was doing. I had gotten so used to hitting the SHUTTER button after making a setting change, I was doing the same for the FEC change - and the FEC change was not taking effect.
Lesson Learned: In addition to checking out your gear for operability before an event, you should also double check the techniques and methods you intend to use ... so you can catch any steps you may have forgotten and get your routine down correctly for making changes on the fly. Oh, and also ... slow down1