。
She couldn't look me straight in the face.
Literal
She [topic-は] my face [object-を] head-on look-able-was-not.
まともに ('properly, straight, head-on') as an adverb here means 'directly / squarely' — 顔をまともに見る is to face someone fully and meet their eyes. The negative potential ~られなかった ('was unable to') turns this into avoidance: she couldn't bring herself to face him. The implication is emotional discomfort — shame, guilt, or perhaps overwhelming feeling — strong enough to break eye contact. Avoiding direct eye contact is socially less fraught in Japan than in many Western cultures (prolonged eye contact can read as confrontational or rude), but failing to meet someone's gaze when expected still signals significant emotional distress.