petunia cuttings

i was potting up all my cuttings today and selected some petunias that have been 'growing top growth' so i thought they had rooted, when i emptied a few out, they haven't got any roots at all, how is this happening? i mist them at least twice a day and they are still indoors in a light area but not hot sun, they have been awaiting roots about 6 weeks ish (i can't actually remmber when these ones were started but doubt it was less than that, i have managed to root other petunia cuttings but these ones.......