I think the area accumulation might be the biggest indicator. If the box only clocked up 8.5Ha in a 15Ha field then there is either something wrong with the radar sensor or it is intermittently cutting in and out of work.
If the radar starts to vibrate when you are drilling this can cause a fluctuation in the reading. You should see this on the screen when it's happening. Alternatively, if the cut out switch has a bad connection, or it's moved on it's mounting and is being deflected as you're drilling, it could be cutting the system out of work when it's not meant to. Do you ever notice the tick changing to a cross at the top of the screen, next to the time, when the drill is down?
If it turns out the other screen you are trying works, you can always do a software reset on the original unit, back to factory defaults, and put in your factors again. That might just clear it out and get it working.
If the radar starts to vibrate when you are drilling this can cause a fluctuation in the reading. You should see this on the screen when it's happening. Alternatively, if the cut out switch has a bad connection, or it's moved on it's mounting and is being deflected as you're drilling, it could be cutting the system out of work when it's not meant to. Do you ever notice the tick changing to a cross at the top of the screen, next to the time, when the drill is down?
If it turns out the other screen you are trying works, you can always do a software reset on the original unit, back to factory defaults, and put in your factors again. That might just clear it out and get it working.