problem with Iphone 5s usable storage

[censa]

Hi, today i've uptaded some apps on my iphone 5s (with 16gb of capacity), like instagram, whatsapp ecc...when i've tried to open whatsapp, i ran into the "optimizing" interface, that warned me that i havent enough avaible space to use whatsapp (so basically i couldnt respond or receive messagges). I got into storage setting and i saw that i have as usable storage just 300Mb...the fact is that with apps and photos i use just 3 gb of spaces out of 12 gb avaible (4gb are used by factory settings ecc), so i should have approxximately 12-3=9gb of usable storage..How it tells me that i have avaible just 300mb? i've tried by deleting some pics/apps but nothing has changed so far..
Thanks in advance for your help, i really dont know where to bang my head

 

[Audit13]

I recommend backing up iTunes, resetting, and selectively restoring apps.

I'm not sure why the phone is reporting such a small amount of free space.

 

[Minorite]

Have you checked the actual free space on your device with Coconut or iMazing?

 

[nieks]

Please go to settings, general, storage. There you can see what has eaten your storage on the phone. For example: your phone might have downloaded a software update.