A correlative conjunction comes in two halves, and each half must be followed by the same kind of word.

The two halves are a frame. Whatever follows the first half sets the shape the second half must match, and the exam scores the mismatch even when the meaning is perfectly clear.

She is not only competent but also willing.

adjective after each half

She not only is competent but also willing.

verb after the first, adjective after the second

Where it bites

Move the first half one word and the sentence breaks. Test by deleting both halves: 'she is competent but willing' must still parse.