One Armature disappears when joining Armatures

I have appended a piece of armature into my current working character file. When attempting to join the armatures, one of them seems to mostly disappear. If I join them the other way around, the other armature does the same. Any idea what is happening? I made the origins the same, scale is 1 on both.

Thanks for looking.

Can you post the blend file please - it may be an armature layer issue? I cannot tell without seeing the file.

Cheers, Clock.

Spot on, it was a bone layer issue.
Thankyou Clock.