Some people have written that they are hit-or-miss with overclocking beyond 2GH/s --without-- changing the resistors.
It may be that the voltage needed to overclock simply isn't there with all hubs/connections/some other speculative answer.
The answer may lie in the fact that with stock resistors the device is working on 0.8v.
If you change the resistors to allow a supply of 1.0v through the device it may overclock with no argument.
Just a thought. Also, I found it pretty cool that Bitmain would supply this kind of information without being asked.
Sorry for crappy answer. I'm sure it's not the answer you were looking for