I have heard lot's of explanations. The one I tend to go with is that there are duplicate blocks in the two files preventing the bind mid action leaving you with no visible reference.
Here is a list of possible fixes. Start at the top and work your way down. They get more bothersome as you go, but I haven't missed getting it to work eventually yet.
- Right click on the referenced file and pick Open Xref. Make some change like zoom extents, save and close. Reload when prompted in the original drawing then bind.
- Purge both drawings.
- Recover both drawings.
- If the referenced drawing has attached references, change them to Overlay and bind individually.
- Detach the reference file, purge and insert as a block.