I've seen primers or other debris under the trigger, which will completely shut down the rifle. Also, a worn or defective mfg disconnector will lock a trigger and not let it "re-set" as we armorers say. Changing the hammer or the disconnector will usually cure this. When we inspect them, we function check them religiously...I usually will put the selector in semi or single shot and depress the trigger and rack the bolt back and let it go, usually arount 8-10 times. That will give you an idea if it is fixed...hope this helps!