I feel bad posting an issue about cron when there are so many out there, but I've read a ton of other issues, and I can't seem to find a solution that works. Here are some of the issues I found helpful:

Like many suggested, we disabled the search module, cleared search tables in phpmyadmin, and even set up a cron on the server.

Here's a bit of detail about the problem:
We have a site that gets a good amount of traffic, certainly enough to allow poormanscron to run. It even seems to be running every 30 mins, as specified in the settings, according to watchdog. However, the caches are not cleared when it runs, and the feed that should be updated by cron running and clearing the caches is not updated until an administrator runs cron manually by typing in example.com/cron.php or clears the caches through the admin menu.

Cron used to run fine, clearing caches and updating feeds like a charm, and it's not entirely clear what triggered this malfunction. For a while, cron was not running at all; when we found the suggestion to disable the search module, poormanscron began running again, but the caching problem remained.

I hope that's enough info, but I would certainly be happy to provide more if needed. Any help on this would be much appreciated!