Some manufacturers set up with software which has been modified from the regular Android version and so do Service Providers. That said, it won't let you set Internal Storage because that is the default setting so you should be looking for Portable Storage.
Anyway, here's a way of taking more control over your device so go through this procedure.
Tap your Apps square and go to Settings. One of the entries is About phone; go into that and tap Build Number seven times. When that's done, you see a message saying you are now a Developer and an extra menu appears named Developer Options.
That would mean you've set the SdCard to be Portable Storage.
I have to point out that there are some Apps are unable to live outside the system folders but when you've set the storage, you will go to Settings>>More>Application Manager, those that come live on the SdCard will say "Move to SdCard". If that is greyed out, that App must remain in internal storage.