Potty training.......

Ok, I promise I've done this before.......practically every job I've ever had has involved potty training other people's kids.  I have successfully potty trained at least a dozen children, usually woking with three or four at a time.  Why then is it so frustrating to potty train one almost 3 year old???????  He understands the concept, he knows that he is supposed to go potty on the toilet.  He tells me all the time "no potty on floor, no potty in car, no potty at store........."  and he hasn't had an accident except at home.  Why then does he sit in the bathroom for an hour with no result and then promptly pee in his pants????  I kow this is normal but I am going crazy here.  I had enough laundry without all of the extra underwear, pants, and socks!!!!