Most of use have had trouble threading a Picc wire @ one time or another. What are some reasons this occurs, besides scar tissue or plaque in the vein?  You visualize a good size vein and enter it without difficulty and get a great blood return, but you are unable to thread the wire. I have tried both retracting stylet  slightly from the vein and repositioning the angle of the stylet but without success. Any suggestions on how to get wire to advance without giving up and making another insertion site?
I like cannulating with a long 20 gauge catheter. Once the catheter is in the vein, the guidewire slides right in.
If using the 21gauge seldinger needle, make sure that:
1. You can visualize the tip of your needle so that you have a very good idea of where you're actually cannulating. If you're looking at the shaft of the needle instead of the tip, you could be cannulating the side of the vessel, or cannulating in a bad spot (bifurcation or valve, for eg).
2. The plane of the length of the needle is parallel to the plane (direction) of the blood vessel, so that you're not creating yet another angle for the wire to turn around.
3. The needle is coming down right on top, in the middle, of the vessel.
Other thoughts:
Once you have a good blood return and you've tried to advance the guidewire but can't, try to bring the angle of the needle down (but keep in right direction)
and also remember that if you take the ultrasound probe off of the arm after you cannulate, the tissue displacement will/may cause the needle to pull back out of the vein some or completely. Have your guidewire immediately ready - right near insertion site - so that you don't have to manuever yourself around trying to find it, possible changing position of needle tip.
Try cannulating without a tourniquet.
Cannulate with the probe in position for sagittal view.
Mari Cordes, BS RN
Mari Cordes, BS RNIII VA-BC
Vascular Access Department
University of Vermont Medical Center