I just got my DA21 back from Pentax hospital for just this reason and now it is sharp (in the centre) wide open. I love it now, and trust it at any f-stop which I couldn't before.
Mine was unacceptably soft everywhere <= f/4.5, but sharp as a tack above f/5.6. I did everything I could to rule out other factors; I was using a tripod, 2 sec delay for MLU, live view for focusing, etc etc. Still soft. Mine was within warranty so I put it in to Pentax service department and 6 weeks later I my lens back from Japan and now's it's performing superbly