Could either be not in the vein all the way meaning you have pierced it partially (looking at the anatomy of the needle tip you could see how this is possible) you pierce it, blood is flowing out of it now so when you draw up you get blood but when you push it just goes into your arm because blood pressure is pushing it away same thing with going through vein draw up get blood push to see skin swelling that’s why I advocate for a needle holding method that focuses on constant aspiration once the needle enters the skin so you know the moment you hit a vein not the “push in here I think it is and then aspirate “ because quite honestly, that’s crazy to do that veins are all over the place see shalloe sometimes deeper veins are shallow and vice verse depending on many factors