Switching to 12/12 lighting is a significant shock to the senses for them. That is why I like to be sure my plants are good and healthy and under no stress when I flip them. They will take off again in a day or so when the shock wears off. When they begin stretching, you will see amazing "growth", but remember it is not true growth. It is the plant saying "hey where did all that lovely light go, I must be in the shade. I need to grow above it." Because MJ is such a high energy plant, it is very competitive and greedy for getting all the light it can get. That is why growing in SOG or having many plants in a space can become more of a hindrance than help to maximizing yield.