I don’t think you need to declare the dependency with sources classifier to the sources. If the sources are available, then Buildship attaches them to the corresponding jar in the classpath, unless the functionality is disabled in the eclipse plugin configuration.
Without an actual example, it will be hard to investigate your case. I’d suggest check and - if nothing else works - modify the sourcepath property of the dependencies in the whenMerged block. You’ll find examples how to do that here.