Probably because your body is used to taking more narcs with the Roxy's. Yes I am sure if you up the doseage of the norcos that will take away the wd's. However, then you will run out of your meds early...so it really won't be worth it! I understand you wanting to make it through your b-day but trust me, now its your b-day, then it is because it is Christmas or New Years. There is never a "right" time to quit. Do what you know is right...
hydrocodone is a weaker narcotic than the oxy that you were taking. Its not a 1 to 1 switch situation. You could very easily be in withdrawal trying to make that sort of switch.....