FSXA Cannot Join Problem

Today I ran into this problem. First time I have encountered this problem. As you can see, the two items are selected, but under Part, the *Join* is grayed out. The body and the blade are already animated. Trying to get these pistons animated also.
Using FSDS 3.5.1 and FSX 10. Thanks in advance.


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Problem was with Ambient.. Went back to parts properties and redid part name to piston and part parent to none on both pistons. Once joined, then I was able to have part name as ambient 2 and part parent to ambient 3. Many thanks for the reply