Hello again. I will give it a try in case you leave us.
Most flagship devices I think allow a generous amount of internal space for apps, and their future updates. Low end devices, though they may have 4 or 8gb total, would only allow sometimes a little over 1 gb for apps, if memory serves (ooh - joke).
I have had 7 to 9 gbs of stock and downloaded apps on my 32gb Samsung Note 3.
I currently have 3.42 gb for "app data and media content" on my 8gb Moto G.
You may have a lot of large games, I don't know.
You can move some of the data from most downloaded apps to sd card, but that doesn't always free a lot of internal space because of the way Android apps work with sd cards.
(Applications Manager - Sd card - select app - App Info - Move to SD Card)
You may have an issue with logcat files.
Have you looked at My Files (open Used Data), and Storage).
If there is a large amount of used storage you cannot account for, you can dial *#9900# (tbc) and delete dumpstate/ logcat files or something like that, it's the second option and is safe.
Sadly most major carrier versions have these access codes blocked.
Come back if these don't work and there is someone here who can help you further.
Facebook
Twitter
Instagram