Root System recovers itself ?

[BrianTS13]

Hi
I'm new to rooting, so I do my trials on my old HTC One S.

I already managed to unlock and root the device.

Currentyl I'm trying to make an app a system app. According to the manuals I read it should be as easy as coping the apk to the /system/app folder and reboot the device.

Unfortunately after reboot, it seems that the system does some kind of reset, as any APK I copied to the /system/app folder have vanished ...

Android 4.1.1
Using:
ES File explorer with r/w access ..

I also tried to use /system/app-mover, but installing the BusyBox fails.

Any clues ?