Without seeing what you are doing, there is a chance you are in the vein when you pull back (register) but then when you push the plunger you push through the wall of the vein, thereby missing.
I have seen someone else do this before--he would draw back and be in. Then he would push the plunger and feel it missing and draw back and still get blood. But see, when he pulled back he was actually pulling the whole syringe back into the vein. He wasn't good at holding the entire syringe still in the vein while only depressing the plunger. I saw this most often on IV coke, as the veins themselves contract, plus the user is shaky. But that one friend, he just couldn't hold it really steady one handed.
Other than that--is it a register of dark, fast flowing venous blood that rushes into the syringe? Or is it a little slow trickle? Slow trickle registers usually mean you're barely in, or you're in a thin branch of a vein, neither of which are a great place to go.
Blood pressure isn't going to make you miss.
Without more specifics I can't speculate more. What I can say is-- get into a good vein with good flow. As soon as you have the tip of needle in skin draw back slightly so you see the blood hit as soon as you puncture a vein. Once you know you are in good, slowly press plunger. Don't check your register a whole bunch of times either--I've seen people do this and screw up a perfectly good register over checking and pushing through vein.
Or if possible have someone else hit you to see it someone steady has the same issue