Eclipse java debugging: source not found

While debugging a java app in eclipse I receive a "Source not found" error in two cases:

• Stepping into a file in a different project which is already imported

• Stepping into a file in an installed maven repository

The files are there, but eclipse won't step into them, instead it shows a button to "attach source"

I tried attaching (which opened a dialog to define a variable?!) and eclipse did jump to the file, but the debugger could not inspect any variables there. Also manually attaching the source for each dependency isn't practical, as in my case there are thousands of dependency files.

I'm new to eclipsejava so an explanation of why this is happening + how to resolve this would help a lot!