Something a lot of guys never consider is how our bodies change over time, you may be using gear, stacking X, Y and Z get great results and experience no negative side-effects. A few years later you use the same stack at the same dose, the gains are not quite the same, maybe better, maybe worse but this time there are negative side-effects involved.
You see this kind of thing in bodybuilding all the time, guys have some success and follow the same protocol with gear, diet and training for the rest of their lives; eventually it will not work as well, changes to the plan need to be made but in their stubbornness they refuse to change, they refuse to really look into the situation and continually bang their head against the wall year after year. They may make some gains but never to the degree they could and for the most part in most cases stay the same.
While there can be many different factors at play it could be a simple adjustment with gear but if you don't have a simple and basic understanding of function you'll never correct it.