Not enough reliable observations
The signal of the tag was not received by enough anchors to calculate a position. Verify the anchor coverage and place additional anchors as needed.
Positioning result is considered invalid
The system was able to calculate a position but this calculated position was considered invalid. This is mostly related to positioning settings: It could be that the calculated position lies outside of a positioning zone, but it could also be that the freedom of movement of the tag is too strict and the positioner considered the calculated position too far away from previous positions, and therefor invalid:
Check your positioning zones, and make sure that these cover the area where your tag is active.
Check the freedom of movement in the positioning settings (Settings → Positioning), and make sure that these is not too strict: Start with a strong freedom of movement, and an unpredictable movement model.