I had the same problem. Then, I was prescribed a medication used to increase dopamine, and adrenaline, production, and now it does.

Not saying this is some trick to make exercise suddenly release a bunch of endorphins, but it very much did that for me, and when I told my doctor about it, she said that was something that commonly reported. It has even been looked into as a performance enhancing drug, by a number of sports regulation organizations.