One of the pitfalls in writing genre fiction is, of course, the dreaded infodump. Who doesn't see one of those wind-ups where 6000 years of history is about to be dropped on the reader and cringe? Or, more often than not, start flipping through pages to find where it ends and the story starts up again.
I do my best to avoid it.
However, one thing that's as much a problem, but harder to catch yourself doing, is the opposite problem: underexplaining. It is, in fact, incredibly easy to do.
For me, the problem stems from, when I'm writing, certain points being SO OBVIOUS in my head, that I forget I would need that I actually have to explain them to the reader. Funnily enough, where that would come into play more was less about 6000 years of history or the complex internal politics of one nation, and more about the simple dynamics between sets of characters.
Case it point, though this isn't my own work, but something I did a crit read on a few years ago. In this manuscript draft, the main character (let's call her "Brianna", though that wasn't her name, but I really can't recall), in a low moment, needing some guidance, wishes she could talk to (to make up another) Rhenna. As I read this, I thought, "Who the heck is Rhenna?" I dug back through chapters until I found the reference-- much earlier (years earlier in the story), Brianna had traveled with a group from point A to point B (as is the way of things in epic fantasy), and Rhenna was a member of that group. However, Rhenna is little more than a name drop. She doesn't have a single line of dialogue. When I asked the author, "Why would Brianna want to talk to Rhenna for advice?" "Well, when they traveled together, they talked a lot, and Brianna really looked up to Rhenna during that time."
In as much as I ever do during a crit session of any kind, I flipped. "THAT'S NOT IN THE TEXT!" OK, I didn't shout it, but I did make my point in a strong manner. The point is, there was all this stuff about Brianna's journey from Point A to Point B, which was three months of time for the character, but two pages of text for the reader, that the author KNEW, but didn't tell us about.
I'm not claiming my own innocence here. Lord knows the trunked manuscript Crown of Druthal is FULL TO THE BRIM of that sort of thing. There I have a ship full of characters in which I pretty much expected the reader to read my mind about the details of the various friendships, enmities and barely-tolerated acceptances between them all.
Frankly, this is a key reason why I think everyone should have a crit group or beta readers, because it's important for someone who doesn't live inside your head to look at it and yell at you for presuming facts not in evidence.