DrMacro: Thanks for the reply. I often use the reference tool you mention, but I find that I very often can't select the sketch I want to reference. The limitations on what can be selected are not documented anyplace I can find. I may (reluctantly) need to look at the source code to figure out what I can't do
The 'copy' tool I was referring to is this:
which I often find useful for referring to sketches. It also has limitations, I'm afraid.
At this point I'm finding it nearly impossible to modify the model to fix the problems. I may need to rebuild it again