Sometimes in assembly file parts might be work part out of your control.
This problem occurs because of the “Allow Automatic Work Part Change” active in NX 12. The command is shown in the figure. Disable this command to solve this problem.
For Example: If you try to create a plane related to a part in the assembly file. The part that related to the plane will be the work part and Plane will be a feature in the Part file. If you want to create a plane in assembly file disable the “Allow Automatic Work Part Change” command.