It happened to me but I didn’t knew it couldn’t mirror it after clone it 
Basically, I made an ear out of a poly-sphere (not a regular ear but one which many details). I did the sculpt until I was satisfied and appended it to the main model where the head is. Resized it, moved it to place and then cloned it. What was my surprise when I try to mirror that ear and this message pops out:
I don’t know what I did wrong. I already did this with the eyes and all went fine and did my mirror, but now it seems I can’t mirror this ear like I did with the eyes… What do you think I did wrong here?
that solved my problem quite perfectly. Thanks for the help and sorry if my question sounded a little silly :o