I'm slightly different from others in the post in that I treat only once at the back of the hive. I have no trouble with the vapor moving through the hive, and I will see vapor at the entrance or along any leaky top bars after treatment. I've also used the two holes method: that works also. I'm just getting lazier.
Make sure your treatment holes are *between*combs and not on one. This may sound silly (and I guess it is) but I mismeasured once. Rolled a 7. treated. 7. treated. 7. What is going on??!! Open the hive and did the Homer "Doh!" re-drilled the hole a half inch off, treated, got a 2. (I guess this demonstrates the importance of testing post-treatment... or just not shooting the OA directly into a comb.
Best of luck!