I assume by "no effect" you mean it's not getting you high, not that it's actually doing nothing. Presumably it's still at least alleviating withdrawal symptoms right?
How is this different from normal tolerance that gradually grows over time? Do you mean it seems like your tolerance skyrocketed very suddenly and you can now take a lot more than usual without noticeable effects, when your normal amount had been getting you plenty high last week? Are you taking any other drugs? Anything else change in your life or lifestyle? Had you been using more Roxies than usual lately? We need more info in order to tell you if it's just normal tolerance or not. 600mg is a lot of oxycodone to be taking every day. Sometimes with opioids people eventually get such a high tolerance that they don't feel anything anymore other than it relieving withdrawal symptoms unless they take an insanely high dose or switch ROAs (neither of which I recommend!). My first guess is that you just have built up a really high tolerance.
Without more info, my first suggestion would be to try to reduce your dose for a while to lower your tolerance.