Perhaps it wasn't a strong enough one. Looking around briefly, she caught a glimpse of Mr. Quirrell himself. Her heart fluttered. She remembered a time where they had been talking. Just talking. Not about their worries for once. Just their futures, their families. It filled her heart with warmth. And she closed her eyes and thought of that memory, that feeling. "Expecto Patronum." It was stated more as a whisper than anything else. But hopefully it was strong.