On my first flight yesterday, after a while, the W/V calculation failed and a red cross appeared.  During the second flight, W/V still did not calculate.  Any ideas as to what the problem may be please?  (Using latest version of SD, on an Android tablet, feed by PAW Rosetta.)

I believe the wind display is the forecast wind downloaded via WiFi/mobile data, not a calculation. If the device hasn't been able to download a forecast covering the flight (e.g. is in aeroplane mode) then the wind instrument is "red-x"d.