[Q] What can prevent S4 (I9505) from OTA/Kies update? topic

Hi :)

My friend has got S4 (I9505). He wasn't flashing his phone with anything - it is original like it was bought. He has got now I9505XXUBMGA firmware (Android 4.2.2) with BTU CSC, which is outdated. System status says that phone is not modified, but despite this, he is unable to do OTA/Kies update. In both cases (when using OTA or when connecting to PC with Kies) he got message "This is the latest firmware". This is not true, latest for BTU CSC is I9505XXUGNG8 (Android 4.4.2).

Questions:
1) What can prevent OTA/Kies from update of unmodified phone?
2) Can software say, that phone is unmodified when it is not true? For example when flashing count is not "0"? Maybe someone flashed my friends phone before he had bought it? Changed kernel or recovery can lead to such problems?
3) Freezing of some Samsung bloatware can lead to locking both OTA and Kies on PC?
4) Are we able to diagnose this phone (how?) and fix OTA to avoid Odin flashing? (I know that there is hard to brick Samsung phone completely, but there are always some small chances that something will go wrong, my friend is not keen on flashing and I'm not so rich to buy him another one S4 when something will fail :p )

Thanks for answers ;)