Could large hiatal hernia be missed on x-ray?

Yes: Hiatal hernias can be missed on xray because they are sliding in nature -- very position dependent. They are more prominent when supine and will be less so when upright.

Yes: A regular chest x-ray should reveal a large hiatal hernia, but if done for another purpose it could be missed. Ct scan can also miss this if they are looking for something else. Very unlikely to be missed on an upper GI contrast study or esophagram. Hope this helps!

Possibly: If large enough it could appear as an air bubble in the chest or hazy density behind the heart. A lateral x-ray usually shows it better. Can still be missed. A large hernia should show up well on a ct scan with oral contrast. If really there and you have symptoms you will probably need upper endoscopy to fully assess.

Assuming a chest: Radiograph, then not usually, especially if it contains air and the chest radiograph included both a frontal and side projection. However, if the stomach had no air in it, it would be difficult to see behind the heart on a frontal radiograph.