as has been stated about a thousand times on various forums here, the ultrasound and xray testing are not accurate when non-calcified stones are present. these tests will only pick up stones of sufficient density. there are calcium stones and there are also bile pigment stones which may not show up on tests. this is not a hard concept to grasp unless you are deliberately trying not to grasp it.
Ultrasounds readily pick up the stones, and in fact can detect the stones as small as 2mm. It is x-rays and CT scans that do not always pick up gallstones.