Does x-ray always show a bowel obstruction/ blockage?

Yes: A plain xray will usually show dilated bowel and increased amounts of gas or air in that bowel if there is an obstruction or blockage. A ct scan would sometimes give a little more detail on the location or cause of the blockage.

Xray of bowels: X-rays show the air inside the bowels. If contrast is given, it will show the fluid. Your doctor looks for gas bubbles in the intestine with swelling of a piece of bowel. X-ray may not show blockage if there is no air getting into the bowel. Contrast will fill up the bowel, but not get through the area of the blockage. Scar tissue after previous abdominal surgery is a common cause of blockage.