What size cathalon are you usually using? I found that the smaller needles were wayyyyy more likely to give me a "delayed" flashback, and by the time that thick blood made it down to the chamber, i'd actually already gone through the vein, but, like you, i'd think, "Great, i am now IN the vein, there is my flashback"
when it was just the delayed flashback which occurs more often with teeny tiny needles.
Go up a size, see if that helps.
some other things i found helped make IV's easier,
keeping a conversation going with patient, at all times. This keeps patient and you both relaxed. Interview the patient, find out what he is interested in, and act fascinated by that, learn from him, keep asking question on whatever it is that he is interested in. I am very good at getting people to talk, even quiet people, and lots of times, the person is so engrossed in saying their answers, as i am taping the IV in, they exclaim, "wow, that didn't even hurt!" cuz i used the most powerful of all meds----DISTRACTION.
For hard to get pts, get a "chuck" (those blue bed liner squares, plastic on one side, papery on the other side). Lay the chuck with plastic side up, under the pt's arm. Do both arms, is not a bad idea either.
Get a warm to almost hot wet towel, squeeze out excess water, wrap arm in that wet super warm towel, then wrap that chuck over the wet towel. Tape the chuck in place.
Tape the ends very well, or use extra chucks or towels, or you'll end up with a wet sheet.
ask patient to open/close fist off/on while his arms are 'soaking'.
Put HOB up, if no contradictions to that.
Find something to do for 5 or 10 minutes.
Unwrap arm. Now, the chance the veins are bulging is increased.
also, my own personal pet peeve about IV starts, is, never ever slap an arm, that drives me nutz to see anyone doing that. Not only is it useless, it hurts, it makes patient tense, and it reddens the skin so much, you might be obscuring visual cues.
gently rub the vein, repeatedly, in downward way. INtermittantly, 'bounce' your finger on it, assessing how full it is, also helps dilate it.
Keep running your finger down that vein, not only will that help dilate the vein, it helps you spot trouble areas like scars, curls, corners, hardened valves, etc. (THIS is when you begin your interview of the patient).
Be a rubber, not a slapper, ha ha!!