yellow leaves

i have a small hydro tray (4 plants) under 4 wks into flower the prob is the leaves have turned yellow and just fall off easily but the buds r still forming with new green leaves and the plants seem ok,the plants are cheesse weed,is this norm or ma doing summit wrong (first time grower)

Is it all the leaves or just the lower ones? any chance of pics?

here are a few,not the best,any advice is welcome :)[attachment=o209189]

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