can't nail (something) down

Unable to complete, do correctly, or be certain of some task or thing. I've been working on my speech for an hour, but I can't seem to nail it down. They couldn't nail down the terms of their agreement. I know I've heard this song before, but I can't quite nail it down.

Common Names:

Name	Pronounced	Usage
Lerato	-	Southern African, Sotho
Agatha	AG-ə-thə (English)	English, Ancient Greek (Latinized)
Nicola (2)	NIK-ə-lə (English)	German, Czech, English
RÍOnach	-	Irish
Bricius	-	Ancient Celtic (Latinized)
Varvara	vahr-VAH-rah (Russian)	Russian, Greek, Bulgarian, Macedonian