Ive taken clones about 2 1/2 week into flower, from my experience they will root but it takes a long time for them to start growing , I have a L.U.I. strain I took while in flower she is sure taking her time though.
The Optimum is from a Healthy vegging other plant... Once flowering starts you are asking plants to revert to their previous life functions and this takes the plant awhile to figure out...
my current grow is a white widow clone taken 3 weeks into flowering. Took AGES to grow but has turned out amazing.
Clone during veg if you can.
Have a look at the WW clone journal in my sig.
Good luck