I am no longer able to make crawlers in Call of Duty: Black Ops Zombies?

If I am not playing an online match in call of duty zombies, I cannot make a single crawler no matter what. I literally got to round 52 without a single crawler and yes I know how to make them. I am experienced at zombies and used to be able to make them, but on my system I can no longer do that for some reason... has this ever happened to anyone else? Or does anyone ahve any idea what's going on??