Why would the heart not be visible on a echocardiogram imagaing, ?

several possibilites. Ultrasound doesn't penetrate air and has difficulty with excessive depth of tissue - thus if you have COPD or are unusually thin or are unusually overweight, the ultrasound will have trouble imaging your heart. If you were born with dextrocardia (heart on the wrong side), us won't find it (unless they look hard!). Finally, their equipment may have malfunctioned. Go elsewhere if you're able.