When FULL-DISKfighter investigates your system for unnecessary files it may encounter files that could be removed, but when the actually clean happens some of the files are locked by the system. This is impossible to check and predict this will occur  before the program actually starts trying to remove them. The reason the files are locked could be because you have a program open that may not use the particular temporary file, but this has temporary locked it anyway. Don't worry if you have a few files left that you are not able to remove, it should be very few anyway. It is a good idea to close all programs before scanning.