make fired by a Junit test does not compile the same way as the ordinary build ?

Here is the problem:

I changed code to adapt to java 1.8: some methods of a Class were moved to default methods of an interface.

everything compiles fine.

but when I start a Junit tests there is a make which is fired and then the compiler pretends that it does not find the methods that were moved .... curioulsy this does not happen on all codes (only on some  modules)

I tried to be sure that all modules were using the 1.8 compiler and were generating 1.8 binaries -> ok

but the snag still persist

knowing what the make does in detail will be helpful to understand the problem

any hint on what to check?

thanks