Have you noticed that as RPGs have progressed there are skillsets and concepts the new books just don't teach anymore? This isn't true in every game I'm sure, but in some of the big ones I've noticed that it happens. Oddly this is most notable when it comes to tools for the Game Master. I say odd because every game that is released should be trying to bring new people into the game. New people, by definition, are new. And yet there are some very basic things in places that are just not taught.
What kind of stuff? The most notable I've come across lately is that 5th Ed Dungeons and Dragons does not teach you how to make a dungeon. It is near half the title, the GM for the game is called the 'Dungeon Master', and it's just not taught.
Now, don't get me wrong, the DMG does have a section on designing a dungeon, but it does not teach you how to build the mechanics of the dungeon. Most of the section talks about the story of the dungeon. The rest gives some ideas of things you might want in a dungeon, but just in a giant list of "here are some hazards and obstacles that could pop up."
There is nothing about how to map. There is nothing about how to set up encounters in the dungeon. There is nothing about how to place treasure in the dungeon. There is nothing about pacing, or the benefits of having multiple paths versus more linear designs.
This information exists online, sure. You can find it. However even there it is fairly spread out. I know, I've been looking for it lately. Even going into Old School Essentials I did not find a lot on how to build a dungeon, but what was there gave more information on how to run one at least.
It makes me curious what skillsets that are core to the games you run are not taught in that game? And how are you overcoming that obstacle?