Android assets - not all folders/files packaged in APK

I have problem with ASSETS folder in android. I put folders and files in ASSETS, and for some folders/files I got FileNotFoundException. Then I printed all folder/file names from ASSETS folder by code, and I saw that not all folders/files are packaged into APK. Why is that happening? All files and folders have names without special characters, example: "Prevoz tereta i lica vozilima/Prevoz tereta i lica vozilima.json"? For some files/folders it works, and for some it doesn't. Does anyone know how to solve this problem? Thanks in advance!