one plant not flowering

So I have a 4x4x6 grow tent with a 240 watt led, I have 3 plants all about 3 foot tall 2 of them have started to flower really well but the last one has not shown any sign of sex they have been under 12/12 for 4 weeks now. Is there anythhing I need to change to help it start flowering? Nutes are ff soil is ffof in 5 gal grow bags