Here's a possible easy fix for when your Linux Mint Update Manager is not refreshing properly. It may appear as below with "Could not refresh the list of updates" displayed in the bottom status bar.
What we're really talking about, of course, is Mint's friendly UI for apt-get, the command-line tool for working with the Advanced Packaging Tool, so there are command-line options for resolving this issue. We're going to stick to the friendly front end instead and pull up Mint's Software Sources Administration Tool.
Notice the "Fix Mergelist problems" option under the Maintenance Panel. After running the Mergelist Fix you'll refresh your Repository Cache and should be good to go.