Courtesy of Mashable:

Have you, like so many other users, experienced problems with Twitter in the last couple of days? Not getting updates from all the people you’re following, but only a handful? The problem seems to be lying with Twitter cache.
Here’s a quick fix for the problem. Simply find some person you’re not already following, follow them and then unfollow them. Refresh your Twitter page and voila, your Twitter cache should now be restored, and you should be getting updates from everyone.
Thanks to engtech for the tip on Twitter. Check his blog, too, it’s cool.