Re: why only low resolution pic save in camera roll ??
If you are talking about 930 / Icon / 1520... Only the 5MP shows in Camera Roll when using Photos App. The high res jpg can be had if you plug into USB and use the File Browser on your PC. If you shoot for 5MP + high res DNG the dng won't show in Camera Roll, but you can open it in Rawer right on the phone.
If this is not your issue, follow Harrie's advice above. ^^^