Folks often refer to woods like oak, hickory and locust as "coaling" woods because they exhibit that property. I like to mix my wood to avoid that. I burn lots of cherry, and since that doesn't form big coals, it is a good one to toss on top of a big coal bed. In general, the denser the wood is, the slower it burns and the more coaling tendency there is. But there lots of BTUs in those woods, so getting the right mix is the secret to getting good heat out of your stove. Throwing split after split of oak on top of a coal bed, eventually the box has nothing but coals in it. Air can't get through it, so it just sits there and doesn't give any heat. If your stove was a forge, you'd just force more air in, but a stove relies on draft to suck air in, so as the flames die, the flue cools and less air comes in, which further cools the flue.... you get the picture.
Wood has a greater tendency to form coals the less seasoned it is, so I agree that your wood is probably not as dry inside as you think. Oak needs lots of conditioning time to become a prime firewood. It's not just dryness that make a wood burn well. Constant exposure to the elements will leave it deeply cracked, which helps it burn faster. Even fully dried wood seems to burn much better after it has had been abused by a few seasonal swings. I buy my wood in the fall, but I don't burn it all then. I pull out the all the denser stuff and stack it separately. I leave it in a sunny location to season for the next year, and it stays uncovered until the next fall. Getting wet and dry over and over all summer long really helps to condition dense woods like oak for the stove. At least that is my experience.
Also, make sure that it is all coals in there. Ash can easily build up unnoticed under a coal bed that is too big and hot to deal with. I've been surprised to find massive amounts of dense ash under not as many coals as it looked like.