I think that error message relates to constraints, not to flow.
I've found the same kind of issue.
If the nature of your pipe network is that some fittings are constrained to only move along the pipe, it is hard to copy that section. Especially if it works out that some fittings are constrained north/south while others are constrained east/west. That would explain why you have a problem on one system and not the other. The system you can copy is probably a more flexible configuration.
I either just let things break, then fix them, or try to copy smaller sections and redraw some of it. Its often just as quick once you get the 'connect into' working.
Re your other post, I do much the same. Insert (or copy) the trap, draw a straight pipe, then rotate it to the right direction.
I also use reference planes and align a lot, especially when I draw the pipe sloping.