A 47-year-old man with heartburn and regurgitation undergoes upper endoscopy, which reveals Los Angeles grade B erosive esophagitis. He is started on omeprazole 20 mg daily. He reports taking the medication reliably every night at bedtime, approximately 2–3 hours after finishing dinner, and denies missing any doses. At a 2-month follow-up, he continues to experience heartburn after meals. Repeat endoscopy shows persistent esophageal erosions.
Which of the following best explains why this patient's treatment has been ineffective?