Thought I finally had pressure pipes figured out - good luck with that! Basically, you are finding that the software doesn't work correctly. Pressure networks are really sensitive to the slightest mis-step. When we have disappearing parts in plan or profile we have never found a suitable solution. You can dref the network into another dwg, or copy/paste it, and it doesn't matter - once corrupted the network stays corrupted. The only true resolution is to start over. It may have something to do with the parts list, or the connection to the parts list? What happens if you switch to a different parts list? We have created a tool palette with dummy parts in it so that we can get the dam plans out instead of wasting our time trying to find disappeared parts. The trick is getting it to label, so we have also created "placeholder" parts on Defpoints that you can label if you can get them to show. It's been a while since I helped someone figure that out, and it was for a disappeared profile view part, so I can't really describe the steps we took for the placeholder parts.
You will encounter problem after problem, all inexplicable, working with pressure networks. Be methodical and make no false moves, setting things up correctly from the very beginning or you'll keep running into mysteries like this.
