yeah, sometimes Sativa dom strains can be a bitch to grow and can sometimes take a little longer to show sex (begin flowering).
just be patient my friend.. just be patient. Eventually the flowering hormones will kick into high gear, and she'll start flowering.
One thing you might want to consider.. in order to kinda 'force' the plant to begin flowering.. is to lower the 'lights on' time.
In other words, if your running 12/12 currently.. try lowering it to like a 11/13 light schedule.. this will help induce the 'flowering hormones'
which in return should kick the flowering cycle into high gear.
Good luck.
peace.