Nero - copying dvd to dvd disappears?

sometimes, not all the time, when i put a dvd movie in and put a blank in to make a copy of it and click on copy dvd, nero disappears for about 2 secs then appears back on screen and i click on burn and it goes thru it things but sometimes it disappears and goes down to my bottom task bar and sits there and doesnt come back up, why is this.?