Two things I can think of: First you get the words, then you recieve knowledge of the words from one of the Greybeards and the graphic looks kind of like absorbing a dragon soul. It sounds like maybe you missed the receiving knowledge part, or that part bugged out. Secondly, are you holding the shout button down long enough?
If it's not greyed out then you should have gotten the knowledge, so . . . Skyrim can be pretty weird sometimes with stuck animations and key inputs not registering. Restarting the game might help.
Yeah I think it is bugged, I tried to reload from a previous saved game and started from there and still no go. I get the Greybeards to give me the shout, it becomes activated, I recieve animation from him giving me the shout, than the quest line says to go and cast Clear Skies at target, and nothing happens. I wonder If the save games got corrupted somehow. This is not the first time it's done this. I had to get passed the quest line for Winterhold by going into control console and setting it complete. No matter what I casted at the mage meeting, he did not move on from the same dialogue. Don't think I can set complete on the Mainquest, no info on how to do that. Looks like I will have to wait for the next patch or Creation Kit to set the quest as complete. I don't want to start all over again, all the hard work and time, won't have time when college starts again next week.