Memory Game

Ramdomizing in memory games

Hi, I´m a teacher from a brazilian basic school, and I´ve created a memory game with the photo and names from my 20 students. I´ve selected the random option, with 6 pairs of cards.  Instead of randomizing between the 20 pairs, it randons only with the same 6 pairs always (that the program selected). It has frustated the student that didn´t "pop up". Is it an error? How can I solve it? (sorry for my bad English). It happens when playing in my account in, and also when students play in the Wordpress, with Chrome, Firefox, both in notebooks or cell phones.