Why have my dental crowns sunk in? (photos)

I had dental implant surgery due to missing lateral incisors about at age 17, now 33. My crowns have become much shorter than my natural teeth, not sure exactly when it started, I am concerned they could be failing. Perhaps due to bone loss as I am a smoker? I have no pain or mobility, my gums do not look to be receding in that area, the crowns are sinking in! I am hoping just having the crowns replaced will be enough. I saw a dentist about switching crowns and she did not seem concerned.