What has changed? Could my building slide?

I set up a club a month ago and it was a tight fit to keep in between the borders as far as width but I can see the green property line on both sides. I own ALOT of land there but it is horizontally. I had no problems until yesterday when all of a sudden 3 parts of the back wall were returned to me by the neighbor. I fixed it and doublechecked I was on my side. Today 4 parts of my club were returned to me by the neighbor. This was all one structure but now the parts that have been returned have changed to no copy, no modify and I cant even try to fix. I don't want to have to put my building back together everyday. Why if it is on my side and not even touching their land can they autoreturn parts of it to me? Is there anythign I can do? I have sent a message to the neighbor with no response.

Did you deed the parts to your group, and does anyone else in the group have permission to move/edit group objects?

I ask because prims turning nomod/no-copy sounds like default next-owner permissions to me, which will happen due to deeding.

After that, an accidental mouse-mishap by another group member may explain your moving walls.

Are they cut prims?

Noone else has permissions, I did find out what happened. Neighbor had been away so hadn't checked objects on her land in a while. When she did a few parts of my building were linked and the main prim was hanging on her side even though the majority was on my side. I relinked everything and all is happy except everything that got returned to me is now no modify/no copy. I have seen this other places also when something was returned through lost & found

They may have been locked when they returned. Try rezzing them and unlocking them.