I have been sick with ureter stone and it was detected by kub x-ray, ultrsound and ct scan. after a time kub xray couldn't detect it. how it could be?

Stone may have passe: A couple of possibilities are that the stone has passed and is no longer there. Another possibility is that the stone is just obscured and not able to be seen. This could be due to overlying bony structures or stool. Sometimes if the image is blurry, small stones can be very difficult to see even if they are still present.

CT more sensitive: Ct is much more sensitive for detecting ureteral stones than a kub xray. Also the stone may have moved from the ureter to the bladder, or may have passed from the body entirely. On a kub, stones can be obscured by bowel contents or by overlying bony structures if it has moved into the pelvis.