A hard reset?
Might work, clear any corruption. Copied from another forum:
This reset will erase your saved Favorites. First turn the unit OFF, then HOLD your thumb on the furthest bottom right hand corner of the screen, push as far into the corner as you can on the screen itself. While holding your thumb/finger on the screen press and hold the POWER ON button until you see the ZUMO splash screen, once the splash screen is up you can release the power, but continue to hold your thumb in the right corner of the screen for another 5 - 10 seconds, you should see a window that pops up and asks "Do you really want to erase all user data?", make sure to answer YES to this window, if you don't see this window, then turn unit OFF and try the above instructions again, ensuring your thumb is in the furthest portion of the bottom right hand corner of the touch screen.
After the reset is successful, please place the unit in an open sky view location, stationary, for a minimum of 20 minutes to rebuild satellite memory.