My weapons are not working?!

I tried to do some Factory Deathmatch games today. However, when I shot people, no damage would be sustained by them, even if I could plainly see that they were getting hit. I only ended up getting maybe 3 kills in two games, and it's not because I'm a bad player! For some reason, my weapons will not inflict damage on others. Occasionally I will inflict a little damage, but it is rare. What is the matter?