Why isn't yahoo indexing our sitemap.xml file more often?

[burtonator]

I've got a sitemap.xml in both Google and Yahoo. I'm pinging Google once every 30 minutes (our content changes often) and they're indexing once every 30-90 minutes.

Yahoo is being pinged too but is only indexing us at MOST twice per day.

Anyone else see this?

Anyway to fix it?