Problems I.Ving Amphetamines

[erebus]

I have been injecting Amphetamines for a number of years with no particular issues but recently whenever I try to inject I encounter the following problems:
When drawing back, the barrel takes in air but no blood (Even if needle is in vein) then as he pulls the needle out there is almost a pop as if the problem is related to air-pressure. (Note that he makes sure that there are no air bubbles in the barrel before attempting to inject).
No idea what is causing this! can anyone help?

 

[wezface]

Are you sure you're not going THROUGH the vein to the other side? This can cause that particular issue with the pop, especially if when you pull it out you get a little red flag on the way out. Also make sure your needles are not clogged.

 

[Crashing]

You're missing the vein, unless your veins are filled with air.