I had the same problem for a while. Then I read on a website that often this happens d/t not getting into the vein fully. Since you insert the needle in bevel up and the tip of the needle is slightly longer than the catheter, it is possible to get a flashback while only the tip of the needle is actually in the vein--not the catheter.This makes it nearly impossible to advance the catheter into the vein without blowing it. A couple tricks I learned: 1. Next time you get a flashback, watch the flash chamber and wait to see if it continues to fill before you attempt to thread. 2. I've also heard that after getting the flashback, its better to decrease the angle of your needle so that the angiocatheter almost lays parallel to the arm, and then advance the needle a tiny bit more into the vein BEFORE you try threading. This ensures the catheter is fully in the vein. 3. For a stubborn vein, you can also try hooking the heplock up to the hub and attempt to "float" it in with a saline flush. This opens valves and carries the catheter through forks. NEVER force it in, and stop immediately if it begins to swell around the site. Unfortunately, after I discovered these tricks, I stopped having to insert my own IVs (our techs mainly do it at the hospital that I work at.) Good luck to you!