Greg Reinacker recently blogged about NewsGator feed retrieval intervals. For me, the money quote is this one:
"One of the more common questions/complaints we get is something about a feed not appearing to update in a timely manner. 99% of the time, it’s actually a problem with the feed."
I can certainly attest to the fact that we receive frequent complaints about a specific feed not updating, and when we check into the problem, it's almost always due to the feed itself. As Greg explains:
"These are feeds which have returned some kind of error, and they are “penalized” for it. For example - if a feed 404’s, it is immediately penalized for 24 hours. A 500 server error? 4 hours. Other kinds of errors (including parsing problems) cause penalties of varying lengths, taking into account how many consecutive errors we see."
Read the full post if you're interested in more details.