Meaning: Succeed in getting, achieve
Example: They are having trouble NAILING DOWN the contract.
Meaning: Understand fully
Example: I can't NAIL DOWN what's wrong with their idea, but I'm sure it won't work.
Meaning: Get full information from someone
Example: I can't NAIL them DOWN about when they're going to finish the project.
Meaning: Succeed or achieve something
Example: I NAILED the job DOWN in the first interview.