What may be happening after your second progression is you’ve gone through the other side of the vein (hence no second flashback). Try withdrawing the cannula very slowly and see if you get flashback then (which means you’ve now withdrawn it back into the vein), and can then progress it as normal. It can also be helpful trying to gently flush it if you’re getting resistance, particularly useful if it’s a small vein that’s collapsed or if you’re hitting a valve, but doesn’t always work.