It might be your subconscious desire to let the bike do the work for you that has your power levels dropping. If you never work yourself hard enough to let you body know that it has to get stronger then it won't get stronger.
I don't recall if you ever stated you had any cardio issues, but if I never let my HR get above 120, I would never feel that I even did anything for my cardio health and much less any real exercise.
If your physician has not told you not to do so, then there is no reason not to occasionally work so hard that you hit your maximum possible HR for a short time. I feel strongly that if you want your body to comfortable work at a particular level, then you have to exceed that level regularly to get it to that level.