This can definitely be a problem that shows up when you put an FA31 on a K-5 - I'm speaking from experience here. It may have been a problem with your other lenses, but for some reason it's so much more noticeable with the FA31. I think
the defining characteristic of the FA31 is that it's difficult to tell exactly where in a scene (that is, at what depth) a subject goes from being in-focus to out-of-focus. It's a wonderful characteristic that few lenses can achieve, but somehow when the focus is off it seems to exacerbate the problem, and your photos come out worse than they would with any other lens.
Imagine what happens with this lens when it is front-focusing. Since your subject is commonly in the foreground, everything in the shot is out of focus! It will make you think the lens isn't sharp, when in reality it's transitioning from a vague fuzziness into the normal background blur. The point is this lens is more demanding than usual of both the camera and you, so you don't want the focus off in either direction. Try to get it right on this copy, but if you can't, send it back.
I'd say it's just as likely it's your camera as your lens, but obviously make sure the lens is OK or return it. Do your best with the adjustment charts, and then make your final decision based on real-world shooting.
I recently learned of an inexpensive adjustment solution you can try right away, without waiting for a product to be shipped to you:
FocusTune
I already have the LensAlign from Michael Tapes Design, so I haven't tried this software solution, but it looks good!