How many weeks/days ago did you change the timer to 12/12?
Or is this an Auto?
Some strains will not show pre-sex well. Normally I start counting up from the bottom of the plant to the forth true node (where the leaves sprout out). You should use a magnifying glass because the pre-flowers are so small. You look for the preflowers between where the new leafy shoot is going to grow out later on and between that and the little horn sticker looking thing (stipules). A female preflower will be long and pointy and will later on have a couple white fuzzy hairs sticking out the end. The males will be round and will look like balls. Sometimes they have a crab claw sticking out. It looks like a fist giving you the finger. Like "I'm a boy and now you are going to cull me, so this one is for you". LOL