There could be issues with the rock, itself:

One notable issue occurred during the carving of Thomas Jefferson’s face. Initially, Jefferson was started to the left of George Washington, but the rock in that area proved unsuitable—likely due to unexpected fractures or poor quality beneath the surface. Borglum decided to abandon that section, dynamite it away, and reposition Jefferson to the right of Washington. This wasn’t so much a "flaw in the rock" as it was a practical adjustment to the mountain’s natural irregularities.

Weathering and erosion also posed ongoing challenges. The granite had been exposed for millions of years, and some areas were more weathered than others, requiring careful planning to avoid weaker spots. Borglum and his team used dynamite for rough shaping (removing about 90% of the material) and then refined the details with drills and chisels, adapting to the rock’s condition as they went.

--Brave AI