I have to have bloodwork done every three to four months. I have great veins, but sometimes, they don't want to plump up like they're supposed to. The last time, my blood just stopped running into the tube, and I had to go back and give again. Sigh.
A couple of good tips to help you and the lab tech, though: Drink a lot of water the morning you're going to give. A 16 or 20-ounce bottle is always good, and it won't affect your bloodwork results. It helps plump up your veins.
Second, if you're not on an aspirin regimen already, and can take aspirin safely,, about three or four days before your lab appointment, take one regular strength aspirin every day. That should thin your blood out enough that it runs nicely into the collection tubes. Works very well for me!