external references not available

Hello,

I am running into an interesting issue that does not allow me use external reference geometry from parts in an assemby as external references.  In this case I have a top level assembly (top_assy) that contains 1 sub assembly (sub_assy1) with parts part_sub1 and part_sub2.  I am trying to create a part (top_part1) in context in the top_assy in which I need to access geometry information from part_sub1, part_sub2, etc. 

I have done this many times in the past, but ran into a problem recently that would not let me access the external reference data.  I was in "edit component" mode for top_part1 and trying to use "convert entities" and "offset entities" from parts part_sub1, etc.  When in convert entities window  I select geometry from the sub_parts, but nothing would be added to the selection window.  In a couple of cases I would get an error that told me I was trying access properties from another assembly and that I could not use it. 

I was able to start from scratch doing the same process and the problem was fixe, but I need to know when and why I cannot use external reference data in certain circumstances?

Any suggestions for configuration or setup with assemblies that would cause such issues?

Thanks