Could be hermaphrodites. A single herm plant can seed all your crop. Always watch new genetics closely, daily checking the branches and buds for any sign of herm/male attributes. If you ever find a plant showing herm attributes, nanners, balls, put a trash bag over it, chop the stalk and throw it away. You might get lucky and not pollinate all your other plants if you catch it fast.
Also, my buddy had a fresh air intake system on his grow and even with a somewhat decent HEPA system he was having seeds in his weed, from outdoor grows. Crazy how fine that pollen is. He switched to co2 enriched sealed room with a bigger AC and hasn't had issues since.