Dungeon fighter online problem?

For some reason whenever I try to set my priority to "normal" or anything higher while playing Dungeon fighter online, my computer always says the operation could not be completed, access denied, and if anyone doesn't know what I'm tlking about its when you press ctrl, alt and delete and you start the task manager and go to "Processes" right click on the game from the list below and go to the option set priority doing this is suppose to help the game run more smoothly, but anyway if anyone knows if they can help plz do