Group Command Stopped Working

Hello, I still use Rhino 5 and all has worked great until today. The group command stopped working on a model I’m working on. It still works if I open another model but not on this one or exported pieces of this model… Any ideas?..

Interesting, if I copy a small amount of objects to a new clean model I can get them to group in the new model and then copy them back to the large model and it works out… But if I copy a large portion of objects to the same clean model, they still will not group.

are any of the parts you are trying to group externally linked blocks by chance?