A lot of new information has come up about mythical stuffs mostly with the premiere of the 7th season of Game of Thrones, one of which is about Dragon-Glass.
If you haven't watched the first episode yet then stop on your tracks 'cause the internet is dark and full of spoilers.
The Citadel is the brain of Westeros and we were taken to the Citadel in the new episode.
There's a mountain of Dragon-glass in Dragonstone and the book mentioning it was in the Maester only area. Now, every Maester in Westeros may have been to that area and have learnt a great deal. Why can't they just tell Jon that "Hey Jon, no need to learn how to make Dragonglass, there is a mountain in Dragonstone, let's go and get it." Or do we just assume that all the Maesters out in the seven kingdoms never went through the books whereas Sam got to that page in just a few seconds.
Also, the Archmaester is told by Sam that White Walkers are real and can be killed only with Dragonglass. So, now do we also assume even the Archmaester doesn't know about the mountain of Dragonglass under the soil Dragonstone? Or is there another aspect of this why the Citadel is not very attentive to what Sam has told them?