Another data point.. I was having the same issue earlier this week, which was annoying. I tried a few of the different suggestions, blowing air, wiping the screen with a cloth, pressing on one corner or another, etc. I do have a screen protector on my phone, one because I was thinking I might return it before the 14 day trial, two, because the protector is matte finish versus the glossy screen. I noticed that the screen protector had slipped, ever so slightly to the right, and a very small amount of the circle for the front facing camera was covered. Nothing was covering the proximity sensor, but I figured I'd realign the screen protector just to see what happens. I was actually able to just shear it over by pressing down and sliding, back to the centered point. I figured it would do nothing, but I've had no problems since then, which was two days ago. Is there a chance that the proximity sensor works with the front camera? Maybe the sensor sends out a signal and bounces back to the camera, or something like that. Mine doesn't seem to have any dust in it, so maybe the dust issue could be relative to both the sensor and the lens? It seems to me the dust issue could be resolved by plugging the headphone jack with simple rubber plug of some sort. I don't use headphones with mine, so maybe I'll craft something to go in there. ( I'm sure there must be numerous reasons why this lock issue is coming up, just wanted to share my (current) solution.