Recently, I've noticed that the dock has been starting to display low-resolution icons in place of the former high-resolution icons for common apps like Stickies, Word, iTunes and Preview. Looking at the .icns file within each program, all copies of the icon are present within the file (high and low resolutions), but the dock refuses to display them, leaving some programs looking like this:

Restarting doesn't stop this behaviour, nor does a killall Dock, nor removing the icon and replacing it in the dock. In Finder, the icons display normally. Does anybody know what may be causing this issue? Thanks!