I've been experiencing the same problem in 15.5. Again, its common sense that 15.5 isn't a stable build so we might experience some bugs like this. If someone else in 15.4 confirm that then it might be a real problem. Other than that, lets wait and see until 15.5 becomes stable.

15.5 already is a stable version.
Maybe it's a problem of the file manager.
You can try if it works with the cp command in the terminal or with the Nautilus file manager.