This is a topic that has me bewildered by my children. I HAVE taught them to write thank you notes. I've purchased neat card sets with nifty designs that they would like, even. Told both of them that if they wanted to continue to receive gifts, they'd darn well better thank people. I've even said to my mother, in front of both of my boys, that since they can't take the time to write thank-you notes, she doesn't need to take the time to give them gifts.
My 19 year old has finally started writing them without any prodding from me. He may be weeks late, but at least he writes them. He finally gets it.
The weirdest thing is they are both otherwise beautifully mannered. They even do things I never taught them, such as thanking me for preparing the meal they just ate. They are required to chew with mouths closed and clear their places at the table, but I never once instructed them to thank me, and they BOTH do it. Very bizarre.