Can't pick new references.

Sometimes, when I edit the sketch of an exisiting feature, I'm not
allowed to pick newreferences. I get the message: "References to
additional 3d geometry may not be made; please select again." When I
use Sketch>References (as per the help files), it does not allow me to
delete existing refs or add new ones.

As far as I know, it only happens when I make a copy of the feature
(i.e. mirror command).

So far, I've only come up with two ways to get around it. One is to
delete the feature entirely and recreate it--which is not good if the
feature is complicated or has children. The other is to use
Edit>References to replace one of the exisiting refs with the one I
want. But, sometimes I can't spare any of the existing refs.

What's going on? What specifically causes this lockout? I'd like to
know so I can avoid it or change my methods.

Thanks,
Chris

 

The easiest solution I know of is to delete the copy and recreate it after
the original has been redefined, someone else might know a better way but
that's all I've come up with so far. Replace isn't always an option, but
that is the only other alternative that I'm aware of.