Need HELP asap !!!

For about 3 weeks I'm having some pretty hard time with my girls ... all are gettin' yellow and it seems some of the lower buds have died...

Background :
1. Strains : Jackie White, White Widow , Dutch Dragon, Indica Mix
2. Lights : 2 x 400 hps
3. Soil : Batmix with perlite
4. Nutrients : BioThrive ( grow and boom ) , BigBud
5. Veg time : 4 weeks / 24 hours CFL
6. Flowering time : 4 weeks now ( buds seem too undeveloped ) 12/12
7. Water PH : 6.8-7.0
8. Pots : 4 L

About 3 weeks ago some of the lower leafs started getting brown spots and turning yellow . It seemed like a nutrient burn so I flushed all the plants but this didn't helped at all as now more and more leafs are truning yellow and some of the lower buds have died . I've also checked the roots and there is no problem there ( all the roots are white and strong ) .

Now I'm in the 2nd week giving them only clear water without any nutes .

Does this look like a N deficiency ?? Should I start with the nutes again ?? If yes , full power or 1/4 of the recommanded dosage ?

And final question ... Can I save this crop or it's already dead ?

I've also added some pictures ...


Waiting for your answers . 10x a lot

U must mix your vig nut + flowering nut for flowering .
Plants pray for N I think ,,,And add some cal/mag or epsom salt

Looks like you are severely undernuting them...and considering you flushed and are only giving them straight water...yeah...you want to actually give them some nutes or just let them die?

thanks a lot guys !!! It seems that I was afraid of burning them when I saw the brown spots ... This is my first indoor grow and never had this problem outdoor ... Just gave them some good food and hope it will be ok from now on. Tomorrow I will also get some cal/mag. Just hope for some better results in the end. Any other advice ???

Thanks a lot again !

honestly if you notice burning tips...all you need to do is stop using your nutes for a few waterings...and just be gentle...the plant will take the nutrients from the soil(if its PH'ed correctly) and it will work itself out

They need more nitrogen man......