Computers don't actually generate random numbers. Usually they have an algorithm that requires a "seed" number (usually taken from the system's clock), that then in turn generates a scrambled decimal from this seed that appears to be "random". This decimal is then multiplied by the number of "sides" of the dice and rounded up, to simulate a die roll. However, if the same seed number is used, the algorithm will generate the same "random" number each time, since it is in fact not random at all.

So if there is in fact a problem in the random number generation, it probably has to do with either the same seed number being used by the program for some reason, or the seed numbers used are not significantly different from one another and thus do not generate a significantly different die roll when multiplied by the number of sides and rounded up.