What Causes Erectile Dysfunction?

Hi Doctor I suffer with erectile dysfunction. I can get very good morning erections but not so when masturbating. Cialis sometimes works amazingly but other times it s like I haven t taken it at all. I have had a cardiac ct scan to rule out any cardiovascular issues. They gave me a calcium score of 18 and said no plaque build up in main arteries. Due to heart issues in family history I am on ramipril to control high blood pressure and also on statins for cholesterol. I have visited a urologist who said my testosterone is normal. He also checked my prostate which is fine. I am 48 years old, 6 ft 2 and weigh 16 stone. This however is muscle and not fat. My diet is relatively good, I don t smoke and rarely drink. Are there any other possible causes of my ed? I am confused as to why I can get amazing morning erections and also why cialis sometimes works very well but not others. Thanks Mark