as far as my experience goes, there is no tollerance build-up to smoked / vaporized DMT. This means, the moment you are capable of using the pipe again, you can get effects.
If it did not work both times you tried there are several possibilities:
a) what you have is not dmt. do you know it is?
b) you are burning it
c) it is condensing in your pipe before reaching your lungs
d) you are not inhaling enough - this is the most probable. Check out smoking techniques on the forum. It's important to really inhale deeply and hold what you have inhaled as long as you can. If you still can, go for a second and third toke. Though if you do it right after the first hit you won't be able to take a second one.
What is your dose?
If you don't have an appropriate vaporizer you may consider using enhanced leaf or changa, which is easier to smoke for most and thus gives better results with less ideal smoking devices.