Answer: Different people have different communication preferences. Some rely heavily on email, while others like face-to-face discussion. Your unresponsive colleague may simply be one of those people who prefer talking to writing.
If this information exchange is more important to you than to her, forget about email and have a personal chat. Accommodating her style will actually save you time.
But if changing tactics is impractical, try asking for her help. For example: “When I send emails requesting input, I usually have a tight deadline and need the information quickly. Is there anything I could do to make it easier for you to respond?” Just be sure to ask this question in person, not in writing.
To be smart about email communication, see Ten Things You Should Never Do With Email.