If the trigger has been gummed up with dust, etc. then you might be able to try some contact cleaner.
The first thing that came to mind as I read this is that it might sound like a suggestion to take the switch apart. Whatever you do (the OP), Don't Take the Switch Apart!!
I took one apart 5 years ago to learn about it, and I still haven't found the spring that flew out into my office somewhere.
The switch is a combination of mechanical and solidstate contacts, with a printed circuit resistor for speed control. Unlike normal drills without EC-TEC technology, battery power does not pass through the switch. The switch is just a low-power resistor that controls a "clock" inside the PWM controller. There is a safety feature of the EC-TEC controller on the C12 drill that prevents the motor from starting if you are pulling the power trigger at the same time the controller receives power (like plugging in the battery). An effect of this safety feature is if the controller sees a sudden jump in the resistor position, it activates the safety feature. This is to prevent injury by not allowing the drill to start unexpectedly during a battery change. (If memory serves me).
As Shane pointed out, your best solution is to send the drill in for repair.
I don't recommend spraying in a contact cleaner through the handle because none of it will make it into the switch body. I also don't recommend taking the drill apart (unless instructed to do so by a service technician) because you will have to cut or remove the clutch position decal.
The best "quick fix" is to simply "exercise" the trigger in and out. If there is a piece of dust between the solidstate resistor and the wiper, this will frequently swipe it out of the way.