are all the pots recieving plenty of water? dry bud on a plant aint good if the rest are still flowering and fattening up. any of the stems on those plants with dry bud showing signs of anything out of the ordinary? ie patches or areas of a differant colour below the dry colas?
is it just the top colas that are dry or whole plant?
there may be differant phenos within the grow that will mature earlier then the others. but let someone with some experiance of this answer, probably a well known member, as im stumped on this unless there are signs of anything out of the ordinary going on that may explain this. wish i could be more help. go go