mic level knob cannot be rotated on DDJ-SB?

greetings!
i have been looking for ways to resolve the loud high frequency noise i hear from this DDJ-SB and found that a particular combination of USB cables (short one and an extension) works better than all the other cables i have (even the gold plated one).

anyway, while looking for a way to fix this issue, i found that if i nudge the mic level knob, the noise gets worse. i read someone in another thread in this forum saying exactly the same thing and that if the mic level knob is not set to zero or maximum, the noise gets better. however, here i hit my 2nd problem, in that i am unable to rotate the mic level knob at all! i don't want to break the knob by forcing it, but what could be causing this problem? does anyone know?

thanks