Does a ct or ultrasound always show kidney stones?

Yes: As limited by resolution. Meaning a stone of and eighth of an inch may be difficult to see especially on ultrasound.

No: Ct and ultrasound does not "always" show kidney stones. The resolution may not be sensitive enough to see very small stones less than 2 to 3 mm. Depending on the thickness of the images on the ct scan, a stone may be potentially missed between the images taken by the ct scanner. Ultrasound may also over-read or miss very small stones as well. Overall though, the detection rate is still very hi.