Another auto flowering genetic freak! I just don't understand the attraction of growing these generally unstable mongrels!
Any way, try going to 12/12 for a couple of weeks to see if that will initiate flowering. If it does, it is probably safe to say you were given a photo period seed or an auto throw back with incorrect genetics, and continue flowering at 12/12. Going back to 18/6 or 20/4 after she starts flowering could induce her to revert back to veging and for flowering to once again stop. It could also be a "confused" auto flower that has had a "bad reaction" to the 20/4 light cycle rather than the optimum 18/6 light/dark cycle or it could also never flower due to an over abundance of nitrogen its whole life, borne out by your plants uber green leaves, combined with poor/unstable genetics that has delayed/prevented flowering so far. Best bet is to try 12/12 and ease up on the nutes a bit.
Hope this helps,....
Organoman.