Every time someone tries to give me an iv on my left arm they blow the vein. why does this happen.

Vein: Your veins may not be readily visible and if they are thin and rolling, they may be poked a few times until gotten into and that puts extra stress on the thin wall that makes it break.

Several things: Could be the cause. Some people have more fragile veins, some have veins much more superficial than usual, and they get punctured easily. Tell them the next time they always 'blow' , maybe they can use a smaller size, or infuse at a slower rate.