FA31mm and K-5

So, on a recent trip to bulgaria, somewhat halfway through my 31mm, has refused to focus to infinity on the K5. But at closer distances it would still autofocus.

In the end I have swapped to manual focus, but seems like, even the pictures that I used manual focus on were a bit out of focus.

Now, is the problem lying with the body, or the lens? The 10-17mmfisheye seems to be doing fine on it...

If, the 31mm needs a fix by the way, anywhere you'd recommend me sending it to?

Cheers
Adrian