I wouldn't worry too much - they're individuals and do things in their own time... not even twins are exactly the same in all things! I've had autos go into bloom in week 3 and others that go into bloom in week 7... just have patience. If you want, you could try boosting the bloom nutes the younger one gets to encourage the bloom development (it won't hurt anything).. but the bottom line with autos is that you just have to let them do what they're going to do when they're going to do it...
Good luck!