It seems to be a memory restriction. I have lots of free internal memory, but cannot move google apk's to system/app folder. In order to achieve that, I had to delete some apk's in that folder - it's like that folder has a maximum or fixed size allocated. I could copy files to the root folder, but was unable to move them to the apps folder untill I removed some system apk's. I also have the issue of being unable to move apps to the sd card.