Right, I found exactly the same thing; when this problem happens, it is not that the sensor doesn't work, it just takes a veeeery long time to respond, I found if I leave my finger on the sensor for several seconds without moving it (needs 2 hands to hold the phone still), it will eventually unlock. My thory is that it just takes a long time to swap the program back in and execute the code. This is a perenial problem with Samsung's version of Android; since my first Galaxy S4 six years ago, I have always had this issue with my Samsung phones, there will be times when apps just become totaly unresponsive for several seconds, and when that app is either some phone unlock app, or the launcher itself, the whole phone appears to freeze up...seriously Samsung, you really need to get your software right, or just start using the stock version of Android (which I would be perfectly happy with). The only reason I keep comming back to Samsung is because the hardware is the best. BTW: I also just updated to the latest version of the optimizer app (available today), and the problem has not appeared all day....maybe it resolved something with swapping/paging, but going to call it fixed until it doesn't happen for at least a few days.
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Happens to me too. For sure this is a software issue, it goes away (temporarily) under 2 circumstances: 1) reboot the phone 2) run an optimize to put a bunch of unused apps to sleep (settings->device maintenence->Optimize now) My conclusion is that this is a caching/swapping problem. If the phoen is loaded with apps, and the available memory becomes too low, things start to get swapped out, and if the system swaps out the fingerprint sensor program (which is running in the background waiting for someone to place their finger on the sensor), then the fingerprint sensor becomes unresponsive. Very anoying. My quick solution for the time being is to addd the "Device Maintenence" widget to the main screen; when the problem occurs, just unlock the phone via another method (I usually use retina scan), and then hit the optimize widget....problem goes away for a while (until the fingerprint sensor gets swapped out again). Samsung should fix this problem by disabling spap-out of the fingeprint sensor code in the kernel.
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