Sometimes when the Anchor locations (coordinates) is not accurate, the Tag will not be able to solve its position from the trilateration calculation. How did you measure the Anchor locations? Can you check if the they are correct?
Thanks for the response, yes this is probably the problem, it was measured using a measuring tape and I am not within the 10 cm accuracy for the anchors coordinates.
I cannot try retrieving data with better coordinates so that will have to wait but this is most probably the problem!