Anxiety or Panic Disorder: This is often the most common cause of complaints of inability to take a deep breath in young individuals with normal pulmonary function tests (PFTs) and imaging. Anxiety can lead to hyperventilation and a sensation of not being able to breathe deeply.

Hyperventilation Syndrome: Similar to anxiety, hyperventilation syndrome can cause a feeling of not being able to take a deep breath due to rapid and deep breathing, leading to respiratory alkalosis.
Gastroesophageal Reflux Disease (GERD): GERD can cause chest discomfort and a feeling of tightness in the chest, which might be interpreted as an inability to take a deep breath.
Vocal Cord Dysfunction: This condition can cause symptoms similar to asthma, including a feeling of not being able to breathe deeply, but PFTs and imaging are typically normal.

Pulmonary Embolism: Although less likely with normal PFTs and imaging, pulmonary embolism can present atypically, especially in young individuals, and is a potentially life-threatening condition that must be considered.
Cardiac Conditions (e.g., Cardiac Tamponade, Constrictive Pericarditis): These conditions can cause a feeling of not being able to breathe deeply due to impaired cardiac function, and while they might not be immediately suspected, they are critical to diagnose promptly.

Pneumothorax (Small or Loculated): A small pneumothorax might not be visible on standard chest imaging and could potentially cause a feeling of not being able to take a deep breath.
Neuromuscular Disorders (e.g., Myasthenia Gravis): These disorders can affect the muscles used for breathing, leading to a sensation of respiratory distress, although they would be uncommon in a young individual without other symptoms.
Sarcoidosis: This condition can affect the lungs and cause respiratory symptoms, but it would typically be associated with abnormal imaging or PFTs, making it less likely in this scenario.