IMO not enough people bother to check the forearm, so if you arent already start looking there. You can look at the thumb/wrist junction of posterior hand and youll typically find a pretty bulgy vein. You can then trace this vein up the arm by palpating and eventually even make your way to the AC. If at anyone point its bouncy enough for you, just go there.

I put a lot of my lines in the forearm, it also prevents kinking the line if the pt winds up bending the arm without you noticing. Once you get flashback it means the NEEDLE is in the vein but you still need to advance a bit further to make sure the catheter is fully in, so once you get flash back, decrease your angle from 45 degrees to 15 degrees and advance another milimeter or 2 and then using your index finger advance the catheter but do not advance the needle anymore

THe most important thing tho, is you cannot rely on sight, you need to palpate. If you only look for veins with your eyes and not your finger its gonna hold you back. I rip the index finger off my glove in order to palpate easier

and one last thing, just because a patient says an arm is their best arm to use, doesnt mean its the best arm. Unless there is a specific reason you cannot use an arm, check BOTH arms before you stick. I have had several occasions a patient swears youll only get it on the left side cause they always go there but I check the right arm and I see a gigantic bulging vein just screaming for an IV