If you look in the Bionic's downloaded user manual, it says not to cover the sensor location (to the right of the ear speaker) with a screen protector. You can't see it, so you have to look at the drawing or be sure to buy a screen protector with a large open area around the ear speaker.
Also, rather than a factory data reset, as one poster suggested, a battery pull would be less disruptive and often cures weirdness, especially if there's been a lot of changes/downloads done. Battery out > wait full minute > battery in > turn on. A FDR wipes your Bionic and you start from "out-of-the-box" status. Only needed for major, major issues.