How often can I safely take Percocet 10/325?

I am in alot of pain lately. I am prescribed Percs/Oxycodone 10/325 twice a day, but I am not getting enough pain relief the medication seems to only be lasting 2hrs at times. I got them filled at a discount pharmacy, but have gotten other things filled there as well with no problems. Is it my tolerance level perhaps? Has anyone experienced similar problems?