I have been prescribed 4 10/325 Hydro a day...my back pain gets so bad I can't just take 4 I end up taking 6.?

I was able to buy some oxycontin 30 that takes my pain away for 8 hrs..I'm not using this med to get high It simply helps with my sever pain... Heres my question, why do you think my pain Mgmt. Dr wont prescribe this for me, (but she prescribes 120 10/325 a month) and is this oxy worse that Narco 10/325