Are you using a controller with trigger stops? If you are, disable them and try again. If you're using a regular controller, please clear your cache by unplugging the console for 30-60 seconds, plugging it back in, and trying again. If that does't fix it, try a different controller. Different weapons have different pressure firing points for full trigger pulls, and if there's an issue making it not sense that you've pulled the trigger far enough to fire consistently, and one of the above may fix your issue.