once a pokemon has had the pokerus it can't get it again
so the only reason I can see as to why they aren't gettin' infected again is because they might have already had
the pokerus, u could try and switch out your team members to see if you can spread it to other pokemon
also they might have already gotten it and it's left them, the pokerus only lasts about 2 days so if they had it maybe
they just got over it.
otherwise I can't tell you why it hasn't spread to other pokemon.