Unfortunately, I don’t much use PBR, let alone simplePBR specifically, and so am not all that familiar with it.
My main suggestion, based on what experience I do have, is to compare your texture-slots to those required by simplePBR–if they’re in the wrong order, or some expected slot is missing, then you might find that the rendering doesn’t come out as expected.
Finally, you might look at the below thread from my own dealings with simplePBR–while perhaps not exactly the same, it might perhaps hold some useful information.