how old is the plant? if it's at least 4 weeks then eximine ALL the nodes with a magnifying glass or even better a 30x jewlers loupe. sometimes you can't see anything unless you gently move the stipule aside and then you find the preflower. i have see sexable preflowers hidden behind a stipule so nothing at all was visible unless it was moved aside. take a close look, you could be suprised what you find.