My DD was the same. Had a lot of trouble stretching her even a few mins. past 3 hrs. But she also never took big 8oz bottles. Always did more like 4 to 6 oz even as an older baby, not sure if that was reflux related or just her style. But, personally, i think if she's getting the ozs, gaining, and is happy and well on a 3 hr easy, then I wouldn't worry.