Not calculating Vo2 Max (and therefore no fitness status)

I got the Forerunner 245 three and a half weeks ago and having been running with it since then . Alas, since then, it hasn't measured my vo2 Max. The last reading was from my former device, the 235. I have gone through all the settings and instructions and still it doesn't measure. Tried removing and re-adding the device also. To no avail.