You just started methadone, right Bomb? It takes a little while, unless at a really high dose, for the build up of doses to occur, hence causing that blocking much more.
The blocking dose in general is also different for certain people. For some it's lower, like 70mg's.. for others, it's well over 100. Just depends. Either way, as you're on it longer, you will notice it blocking more and more as your system becomes saturated with methadone and the doses stack up on top of each other.
Clearly though, you're still having cravings to use.. which is indicative of a problem. Whether your dose isn't high enough, or your mindset just isn't right, something has to change for the methadone to be successful, ya know?
I've said this before in other threads. Often times, it takes only an extra 5-10mg's to make a huge difference, and get you over that hump, to the point where you are stable and the cravings go away.