Rotation constraints will not duplicate!

I have an assembly made of 2 parts, both are circular wheels. I have made a successful assembly with the two parts where the two wheels are constrained to eachother all around. One wheel rotates around the y axis while the other rotates around the z axis at the same time, and because they are constrained to eachother they move as one. I have used a joint rotational constraint which has been working fine.

Here is the problem. This assembly animation works perfectly, but if I save the indivudual parts under a different file name, without changing a single thing to the part itself, and then create a new assembly with the new file names, same parts, and create the exact same constraints, it will not work! it will only work with the original part file names?? I have duplicated the assembly several times using the original file names, and each time a new assembly is created with the original, it works just fine. but if i save it to a different name, it no longer works.

Does anyone have any idea why?? I have spent hours trying to figure it out, go over any mistakes, and there is none. It is just simply changing the file name. Please help. I need to change the file names because i need several parts.