Here has been my procedure:
1. Open 3D assembly that corresponds to 2D schematic.
2. Associate 2D parts to 3D parts using the Electrical Manager.
3. Dismiss warning that the part you just associated is not checked out (because you have literally 273 other parts that also need to be associated and none of them are checked out and you don't have time to deal with that message 250+ times right now).
4. Associate 272 other parts.
5. Save the assembly.
6. Close the assembly.
7. Open the assembly.
At this point, NONE of the 273 parts that you just spent all day associating are associated.
8. Become very frustrated.
9. Wonder why you have write 273 ECR's just to check out the parts required do an electrical routing.