What makes the Echo tick?

This cursed girl, why would she not comply?
Even his most advanced 'tools' could not lift her heart, no matter how often he tried.
Of course the girl calmly waited until he was done, with that infuriating little smile across her lips, as if to say 'go on, I'm waiting'.

Ah, he could see it so clearly, the beautiful ticking mechanism her heart would make.
But then, something was wrong at the same time.
He couldn't put his finger on it, there was something alien about the whole wretched thing.

Was that perhaps the reason he couldn't convert her?