There are clinical guidelines (similar in most states) as to who and how many attempts are recommended at cannulation before escalation to a more senior clinician. I believe the recommendation is no more than two attempts before it should be kicked up the line.
EMLA cream is helpful if they don't mind you bringing your own. Get them to put a heat pack on for 10 mins prior to trying and drink a bucket load of water before hand..
If all else fails sometimes you just have to jump up and down and say No until they get someone else or the U/S machine
xoxoxo