It's USB, you say, so it can't be a parallel port or cable problem. As long as Mach3 has the correct pin and you are also looking at the correct pin (not always trivial - I use a CSMIO controller and the pin numbers in Mach3 are not the same as the terminal numbers on the controller!) then it does indeed sound like a controller issue. Good luck!

You could also try JazzCNC's tip - it's not clear why this would work, but it is a Chinese controller so logic doesn't always play a role