You are unable to attach Xrefs to your drawing, or ...
Your Xrefs have detached from your drawing after an event such as an AutoCAD crash. You may see an Unreferenced note after each Xref in the Xref Manager.
This type of issue with Xrefs is often the result of drawing corruption. One or more of your Xrefs may have become corrupted.
Are you having this issue when trying to match a design to California Department of Transportation (CalTrans) stndards – specifically, when using CalTrans linetypes? If so, here's what to do.
The easiest way to test for corruption in your Xrefs is to check the file size of each Xref drawing. Most CAD drawings tend to me no more than 1 to 3 MB in size. If one of more of your Xrefs is 3 MB or larger, the file may be corrupt. Or, if the file size is approaching 3 MB, and the drawing doesn't seem to contain enough linework to merit that size, you may also be dealing with corruption.
We created our Nuke tool to combat drawing corruption in your Xrefs by cleaning your files of Proxy Objects, DGN linetypes, and other junk that can wreak havoc on your drawings. We recommend running the Nuke tool on each Xref before attaching it to your drawing. For more information, see our Nuke tool page.
In this case, you can run the Nuke tool on each of your Xrefs and then re-attach them.
If you run the Nuke tool on a drawing, you will delete all layout tabs in that drawing. If you need to maintain your layout tabs, you can complete a manual drawing cleanup.