Echoing other posters above. It's quite possible you are not giving your body enough time to rest before you exercise the same muscle groups again, especially since several of the routines you describe above are meant to get the muscle to failure. The older you get, the more rest time your body is going to need before working out the same muscle groups again. That may fly in the face of what you've been taught about lifting, but it is a reality that must be planned for if you want to continue experiencing gains.