My Wife's pixi quit recognizing the Touchstone and would not charge. She has two TS covers, and neither was working. So I contacted palm (chat), and we did some troubleshooting and finally got it working. I thought I share what we did with fellow precentralists.

1. Remove TS cover.
2. Plug usb cable into a power source and plug into pre/pixi. If you're using
your computer to charge device, make sure you select just "just
charge"

3. Remove battery for about 2 seconds, then put back into device.
4. Now that device is power by battery, Unplug the usb cable from device
and put TS cover back onto device.
5. open "Device info" app. Tap the upper left corner of screen to bring down
the app menu, select "tests", then "interactive test" and perform the
"power test" (under power test, select "touchstone").

Hope this solve your pre/pixi touchstone problem. If it doesn't, then it's more than likely hardware related.