The gallbladder changes its volume through the day being the largest in the morning and smallest in the evening. It then increases in size. Only certain types of stones will show up on ultrasound, x-ray of ct. The soft immature stones which have the consistency of a jelly sweet are just a capable as blocking the exit of the gallbladder as a hard stone. I had a similar pattern of pain 4 hrs after eating and I had to adjust my diet until I got the offending stone out. It was brown in colour and hard and I have it in a specimen tube, but it didn't show up on the images from CT and ultrasound. Its shape looked as if it had been wedge in; flat. It finally came out on its own not during a flush. It doesn't mean that my flushes and prescription for UDCA didn't play a role. After it was ejected I had a bonanza flush and the pain went after several weeks when the gallbladder got back to working fully. I also switched to morning flushes and change the protocol to be similar to the procedure used in China to remove gallstones.
A mixture of dedication, determination and desperation got a result.