Adblue - Scan says YES but dash computer says NO? - GLC250d 2015

[AB Australia]

Hi All,
After getting the dreaded 800km to stop message my local mechanic has cleaned all elements of the Adblue system, even cleared the little feed cup on exhaust and put 9.5l of AdBlue back into the tank. The scanner can recognise the fluid levels (9.25l in attached picture) etc in the ECU so we know the optical sensor and everything works but the message is not registering on the dash warning?
He reset the message and it flashed up with 9l of Adblue and then went back to "cannot measure fluid level" - any idea why the message is not reading when the scanner can see everything is ok?

 

[onefortheroad]

When I put ad blue in my GLC it can take several miles of driving before computing range . It may calculate with just engine running , over time ??