If neither a colonoscopy or upper endoscopy detects a physiological problem, could the cause of constipation lie with the small bowel?

No, not likely: The small bowel would not be a source of constipation. Also a colonoscopy does not indicate colon physiology, it only evaluates for visual abnormality of the colon mucosal lining. Colon physiology is studied by anal muscle manometry and x-ray study of colon motility with swallowed colon markers (as an example of some tests).

Wrong approach, Jim: There are a variety of causes of constipation. Only in a small minority of cases does endoscopy on either end show the cause. If drinking plenty of water, a stool softener, and roughage in the diet doesn't help, then it's time for a thyroid check, porphyria screen, Chagas? dairy / cheese in diet? meds? Pb poisoned? functional / irritable bowel syndrome? Read up on these.