As suggested above, your needle lock nut may not be tightened sufficiently.
Another possibility is that your airbrush may have been disassembled for cleaning and then incorrectly reassembled.
When you draw back on the trigger, the action should be smooth and positive. If there is any trace of binding or the feel of metal grinding on metal, it may have been incorrectly reassembled. When you say the "little device behind the trigger", this is the lever which acts on the mechanism which moves the needle. The curved part of this lever should be towards the back of the airbrush. If it is reversed (though it's difficult to do this, but I have seen it done), it will result in binding of the trigger and will not move the mechanism to any great extent, if at all.
When you look at the rear of the trigger, the tab on the "back lever" should be hard against the stem of the trigger. If it is reversed, there will be a space of several millimetres between the tab and the stem of the trigger.
If this is the case, withdraw the needle from the back end of the airbrush by loosening the lock nut and pulling it out. This will release the trigger button and the lever behind it (the needle passes through both of these parts). Remove the trigger and the lever. Reinsert the lever with the curved side towards the rear of the AB (you may need a pair of tweezers to do this). Reinsert the trigger button, making sure the hole in the stem is in line with the axis of the AB. Reinsert the needle, tighten the locknut and check for correct action.