Hey Mark
Thanks for replying.
The strangest thing has just happen. After powering down my drive and Mac Book for a few hours and opening up the project, all is well.
I had already checked your suggestions Mark, but still didn’t work. In fact, it worked OK when switched to Approximate.
How weird!! I have noticed that several others have been suffering the same frustrating problem, but I wonder whether theirs was permanent.
I wonder if anyone can figure out why that happened?
Thanks again Mark for your help.