I've only been using Adobe Flash CS6 for two months, so it's all fairly new to me.
However, starting yesterday I encountered a very frustrating problem: when I go to insert a keyframe (F6) it inserts a blank keyframe instead. I am unable to insert a non-blank keyframe, and have to manually copy the previous frame over. The same thing happens even when I don't use the hot key.
there are other ways to insert a keyframe.
regardless, you could try resetting your preferences. from suhas yogin:
Hold down Shift+Ctrl+Alt (Win) or Shift+Command+Option (Mac) prior to launching Flash Professional. If this interferes with an OS dialog, hold this keyboard combination immediately after launching Flash until you see the prompt to reset preferences