What could be the reason for hitting a vein during venipuncture but not being able to draw blood despite some repositioning attempts?

Being near a valve/“leaflet”. You’re in but then you pull back (to draw blood) and you’ve sucked the valve leaflet within the vein and your catheter is blocked. A trick to float/advance past that point or pull back.

Sometimes the vein collapses, sometimes they “blow”, if you go go through the vein you will get a flash of of blood but nothing more, low blood pressure. I was a phlebotomist for 5 years before I became a nurse-from experience the best thing to do is just pull out and find a new location.