Why Is It So Difficult To Buy This Medication. It

Why is it so difficult to buy this medication. It was first prescribed for me by the Cleveland Clinic after heart surgery and I have been using it for over 15 years. But Walgreens and other pharmacies no longer carry the brand name Rythmol. I have tried the generic three times but each time my heart has started to beat erratically. Why is it so difficult to buy this medication. Thank you. Bruce Jacob