Hello my friend,
First of all I need to clear a common misunderstanding, a Stomach is NOT the whole abdomen, the stomach is at the higher left quadrant of the abdomen, therefore an intestinal endoscopy cannot diagnose the diseases of the stomach.
After clearing the misunderstanding let's rephrase your question to "is intestinal endoscopy able to detect all Abdominal problems?"
And the answer for that is NO. You cannot see liver, spleen, kidneys and other abdominal organs via intestinal endoscopy, thus you can only detect problems which are located inside of the gastriointestinal tract without reaching the duodenum.
Thank you for your important question, and I hope my answer satisfied you.