Remember, it's all about the flour
As an Alabamian, biscuits are something I grew up on and never thought about all that much. That is, until I came to New York City and learned that for many bakers, they're as intimidating as something like pie dough is to me. And to be fair, there's nothing as sad as hoping for fluffy, perfect biscuits and ending up with a sheet tray full of dense little hockey pucks. It's infurating! There are a lot of reasons that you might be messing up your biscuits, but luckily, it's an easy problem to fix. Here's how.
You're Using the Wrong Flour
This is the number one culprit for failed biscuits. In the South, soft white winter wheat flour, White Lily, is very common, and great for making biscuits. Outside of the South it's difficult to find. In most stores, you're going to find hard wheat, which just doesn't work as well. There's science behind it as well, as Amanda Mull recently pointed out in The Atlantic: The soft wheat has weaker gluten and less protein, allowing the leavener to trap more air and the biscuits to rise. If you're truly dedicated to biscuits, you can order White Lily to your house. I've also had good luck with a mixture of half all-purpose flour and half cake flour to approximate White Lily's structure.
Your Ingredients Are Too Warm
Like pie dough, you want your biscuit ingredients to be cold. You want the butter to be cold, the buttermilk to be cold, the bowl to be cold—you get the picture. You can throw everything in the refrigerator beforehand, including the bowl you're using, to make sure. You want to cut the butter in, but you don't want it to melt. The cold butter gives biscuits that flakiness that you want. And if your dough gets too warm, you can throw the whole thing in the fridge or freezer for a minute.
You're Overworking the Dough
If you roll the biscuit out too many times, or overwork the mixture in the first place, you're going to encourage the formation of too much gluten which will make the reulting biscuits tough instead of tender and flakey. You want to make sure that all the ingredients are incorporated and then stop.
You're Using a Mixer Instead of Your Hands
If you're making a very large quantity of biscuits then a stand or hand mixer might be a big help, for sure. But if it's just one or two batches, using your hands is vastly preferable because you can better feel the dough coming together. Plus, it's easier to overwork the dough in a mixer, resulting in tough biscuits.
You're Twisting The Biscuit Cutter
You don't necessarily need a biscuit cutter—you can make biscuits square, like Kelly Fields does, and avoid this problem entirely. (That also helps you from overworking the dough by rolling it out over and over to cut round shapes from it.) But when you cut out a biscuit and you twist the cutter, it seals the edges, preventing you from getting those super-flakey sides that are so delicious. Pressing it down once and extracting it means that you avoid that problem.