sexing problems?

Hi there, Im new to this site so this is my first post, Ive been growing weed for about 5 years now and have always had a good crop (even the first one) I am having problems sexing my latest batch of plants. There are supposed to be white widow, but as I was given the seeds by a mate im not quite sure.
There doing really well, 3ft tall, bushing out and smelly as hell,but I cant seem to get them to start flowering!! Ive been giving them 18 hours light under a 1000w m/h lamp. Ive been giving them 12 on and 12 off for the last three weeks (at least) and still nothing.
Before when ive done this it has been easy to tell after about a week or two. Anyone got any suggestions?
Cheers.

maybe u could change to 10/14. that will make them think the days are getting shorter and force flower them.

if i was u, i would try this, but if it doesnt seem to work after a week or so id ask again and maybe think about switching back to 12/12.

are u givin any nutes or anythin? indoor or outdoor?

by the way, welcome to RIU

welcome..

id say check for light leaks, this is prob why they are not flowering, when the lights go off stay in the grow room for 20mins and see is there any unwanted light getting in..

keep lights on 12/12.. if not workin on that 10/14 (this is when they are already flowering and the ripen faster) will b the same..

give nutes with high P for flowering... .... NPK