Actually most people experience quite the opposite and lose sensitivity. I've been OPing for 2 years AND I drink distilled water...so that makes me think there might be something else to this. Could be the oil you are using? Are you adding anything to it, like Oil of Oregano or Peppermint?
Are you brushing your teeth with anything abrasive, like baking soda? Do you use a mouthwash? Are you using anything new in your routine? I read that humans experience the most sensitivity between the ages of 25-30. Are you stressed in any way? Grinding your teeth in your sleep can lead to sensitivity.
I'm trying to remember back to when I started. I could not drink anything acidic because I would be up all night with my teeth throbbing. I remember it being about 3 months into OP that I noticed that I could drink anything I want. Just my experience.
I don't know what to tell you here except to possibly change your oil if you want to give it another shot. I just hate to see anyone give up on this for the exceptional benefits it provides.