Yet another case of "Eclipse debugger can't find source"
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I have been through all the questions on S.O. of the "Eclipse debugger can't find source" variety, but still have no idea what is causing my problem.
Basically I have 2 java projects in eclipse, project A and project B, and A is on B's buildpath. A unit test in B instantiates a class ClassX that resides in A. When highlighting the ClassX constructor call in the unit test in B and pressing F3, eclipse does indeed open the ClassX source in A. However as soon as I try and step into this same call in the debugger, I get the "source not found" window. 
If I click "edit source Lookup path", Then I see that the default path already includes the src folder in project A, where the ClassX source resides.
Furthermore if I click the "source" tab in the debug configuration used to debug the unit test in B, the default source lookup path there too contains the src folder in project A.

So why can't the debugger find the ClassX source?

Any suggestions would be much appreciated!