it happens if your under a certain distance, i'm not sure what it is, i think like 3m, also if you get it into the opponant then it kills w/o exploding, here's a little trick...get one man army pro, then when you run out of n00b t00bs you can switch to another class with them as many times as you like lmao, IW are so stupid ^_^
Or you can just use a scavenger to loot more, I think one loot gives 1 grenade launcher, I find those annoying as well, most of the time they do not explode on impact even when I shoot at the wall rather than the person himself.
I've found this as well, it's driving me insane as I'm trying to get expert gun challenges and it's a lot harder that it would have been in Modern Warfare. Most of my grenade launcher kills have come from firing into the air at the beginning of the game, as even a near direct hit doesn't seem to do much damage. You either hit them directly without detonation for a one hit kill, cause a bit of splash damage or nothing happens. Thrown grenades seem more effective but sometimes mine don't go off. I think when you fire grenade launchers angled too closely to the ground they skitter a bit without detonating. Point being, don't worry - you're not the only one.