keep light out of flowering chamber for full 12 hours, dont stress your clones or plants alot of stress will produce hermies...most common interupted flowering dark period
its caused by stress on the plant and possibly genetics
try to keep a good stable environment for the plants to grow and you shouldnt have any problems
i know if you have a female plant to start with and it goes through way too much stress it can turn hermie so it will produce seeds to try and survive. otherwise i think it can be hereditary also