u r using soil and peat moss as your medium that  don't sound good 2 me but then again you said u don't have a problem until flowering you may have used up all your nutes out of the soil  have u check your PH pin u need 2 add flowering nutes do u water/feed until u have run off  check out growweedeasy.com lot of good advice u can learn a lot ....  my 2 cents