Holding the tool at the same angle for all nails may induce these conditions; as the nail turns closer to the skin, it becomes harder to fit the lower blade in the right attitude under the nail. When cutting a nail, it is not just the correct angle that is important, but also how short it is cut. A shorter cut will bend the nail more, unless the cut is even on both top and bottom of the nail. Causes may include:

Shoes causing a bunching of the toes in the developmental stages of the foot (frequently in people under 21), which can cause the nail to curl and dig into the skin. This is particularly the case in ill-fitting shoes that are too narrow or too short, but any toed shoes may cause an ingrown nail. Poor nail care, including cutting the nail too short, rounded off at the tip or peeled off at the edges instead of being cut straight across. Broken toenails. Trauma to the nail plate or toe, which can occur by dropping objects on or stubbing the toenail, or by the nail protruding through the shoe (as during sports or other vigorous activity), can cause the flesh to become injured and the nail to grow irregularly and press into the flesh. Predisposition, such as abnormally shaped nail beds, nail deformities caused by diseases or a genetic susceptibility, increases the chance of an ingrown nail, but the ingrowth cannot occur without pressure from a shoe.
Intractable may refer to:

Intractable conflict, a form of complex, severe, and enduring conflict
Intractable pain, pain which cannot be controlled/cured by any known treatment
Intractability (complexity), in computational complexity theory