I'll try to make my story short but....
Work on a file for several hours....open, edit, close.... come back the next day/week and work on it some more.... possibly for several weeks then all of a sudden I can't connect new plumbing lines to the ones already in the file.
The only work around I've found is to place a fitting where I will want to make the connection then draw the new line backwards from there. However, the new fitting ends up just "sitting on top" of the existing line, it's not actually connected.
"objrelupdate" does nothing (I've found that works when I have similar issued with ductwork)
ALSO, This happened to 3 of the drawings all at once that I was working on yesterday SO I think it is something to do with the program not the drawing. It used to happen way back in 2006?2007? quite regularily but then not again until yesterday.
Have you tried using the node o-snap? I have done this when this happens. From what I am reading here I am not alone. Also try a copy all into a new drawing. I think it is the drawing file and not the parts that start to lose connections.
I had that happen also, I found the problem is that the Routing preferences add an odd pipe routing named temp. If you go into your routing preferences you'll see it, just delete that routing preference and everything will go back to normal.