You'll get used to feeling the vein "bounce" when you palpate it. Practice on GOOD veins first; close your eyes briefly, and gently push down, then let up; push down, then let up. Do this a few times on the good vein, then move your finger(s) to the skin beside it. Compare the two areas. You'll feel a difference in the response under your fingers (the bounce). Soon, even if you don't see a vein, except for a faint blue streak on the skin, you'll be able to feel one, and somewhat gauge how deep it is.
Regarding the "pop" you're supposed to feel: since we changed to the retractable needles, I no longer feel a pop when I pierce the vein. Go figure. You have to rely on your flashback. Remember too, as soon as you get a flashback, advance the needle a millimeter or two more, just to be sure the bevel is totally inside the vein, before sliding the catheter into the vein.
Too, always apply traction on the skin distal to where you're going to start the IV. It stabilizes the vein and minimizes trauma to the vein. Makes it easier to start the IV!
Good luck, keep trying!