Error Binding Xref

Does anyone else get this - many if not most of my drawings report "error binding xref" when I try to bind them, although they appear to bind until the file is closed and reopened - then sometimes they just evapourate from the drawing. They Insert OK but in all instances the files which report the error do not show the xref/block when I forward them to Autocad users, just the right file size and/or "listable" xref. Weird. So I have to Bind/Insert/trim/purge each xref each time I send it to someone else.