i used to have this problem.. VERY FRUSTRATING!! and the more frustrating it gets, it seems like getting high is an impossibility. just don't worry about it and try not to think about getting high while u smoke.
i remember when i started it was like the first couple of months i never got high and my friends would be ripped! i was just like what the hell? and of course it didnt help that i kept going 'am i high? am i high?' u kno?
try this: have a couple beers so u have a good buzz goin and be at a social event or something and blaze cuz that way ur not really even thinking about getting high ur jsut tlaking to the people and feeling the alcohol buzz then it is possible that the weed will sneak up on u.

good luck man i kno it can be very frustrating but it will come!