IV insertion tip: When the cannula won't thread off... If you go to start an IV and the cannula won't thread off, it may be because you didn't advance the needle enough. There will be blood return once the needle penetrates the vein, but if the cannula isn't inside, it can be difficult to thread it into the vein. If this happens, you can usually advance the needle a bit more, and the cannula will be inside, allowing you to easily thread it off. More IV tips here: https://www.youtube.com/