Why does my contact lens blurr at the end of the day?

Every now and then, I would switch from wearing gles to 2-weeks disposable soft contact lens. I always keep my contact lens clean and follow strict hygiene procedures (I am an optical student after all) when putting them on or storing them. And they're great, up until night time that is. After wearing them for some 6 hours span, I start getting a build up of oil or something and it blurs my vision...as if a thin layer of oil is covering the surface of the lens. Even with a freshly opened pair of contacts, my vision always gets blurred after wearing them for awhile. And yes, I am aware that they could be dry. Lubricant eye drops does NOT fix the problem. So why does this keep happening?