small buds!!!!!!!!!!!

guys my buds are in 6 week of flower and they are very small -everything was good-humidity,ph,temp,co2 everything but they just wont grow!!!!!!!!!!!!!any ideas? im using 600w hps soil 100l pots-4 plant 2 are great and 2 are fucked!!!!! why??? it just killing me all 4 plants had same conditions and 2 are ok and 2 are fucked!!!
 
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