I have a user with a water pipe network where null structures are used where pipes connect. The null structures will not move, nor can you swap them for other structures. If you create a new null structure it will respond once or twice and then it starts to act like the others. When a null is selected it seems fine, but when you try to move it the pipes disappear and the structure does not move, a regen gets the pipes to show up again. Any thoughts? We were in 2011 and have moved to 2012, same issue in both versions.
This is just a thought, are rules applied to the pipes and null structures?
If so, make a new rule called, "No Rule," and delete all the rules in "No Rule." Apply this rule to you pipes and null structures, you should be able to move them.
Hope this helps.