Attach sources does not seem to work

What is the deal with attach sources?   Is there some magic sequence of steps to get it to "take" ?

An example here: i added jackson-json.<version>.jar to the project (systemPath in maven), that works fine.  Then when clicking on code that uses it, "Source not found". Which is also fine. Then I click on "Attach sources" and select jackson-jsonpath-2.1.0-SNAPSHOT-sources.jar.   Well, no error message. And also no effect: the sources are still never found.

That was an example. I have seen this pattern repeatedly. I'd be happy to learn there were some additional step required (but not obvious or documented) to fix this issue.

thanks