Before going to med school I was a nurse and CRNA. I remember to this day, early on, going three months without a successful IV start. Now I get calls when no one else can get a vein, so, like people have said, it is a learnable skill.
I would add to what has been said, after 35 yrs experience, that you probably need to advance the needle after the flash of blood, but only a millimeter or two.
Also, when first moving the needle toward the vein, move slow. I see people ram the needle around and wonder why they missed the flash of blood. Some pts have a really slow flashback into the hub and when moving the needle quickly you might miss it. So slow down a bit. I also agree that the angle of attack needs to be parallel to the skin, never at a 30-45 degree angle like they used to teach. In fact, more importantly, the needle should be parallel to the long axis of the vessel you are trying to enter, either vein or artery.
Also, more people get in trouble when the get a nice flashback then make their next move to pull back on the needle as they advance the cath. It should be: advance the needle a couple of mm, then advance the cath first, holding the needle still, then remove the needle.