Hmm Xperia Z2s have had some issues, alot of them have had faults that can be fixed through repairing using Sony PC Companion
I believe newer firmwares should fix alot of bugs that are actual firmware issues like such.
Things like this arent caused by the firmware but instead your firmware or posibly hardware
-posible fix- use sony PC companion to repair
Or use FlashTool (one word) to flash a FTF file but i recomend the first one if your not so versed in using semi advanced tools
Sony Xperia Z2 <-(id like my phone back pls its been 1 month XD)