i had 2 cockatiels died

by bong vitug
(san diego california)

i had 2 cockatiels died on me just this week, always sleeping.. i notice they were always sleeping, for about a week and never thought they were sick. well one day my first cockatiel died, i thought it was sick. so i bought another cockatiel then in a couple of days my new cockatiel died too, but my other 2 cockatiels in the same cage are fine. what could have cause my 2 cockatiels from dying? please help.