Having Palette 2 or Canvas plugin enabled breaks plugin Software Update in Octoprint 1.5.2 (perhaps all of 1.5, but I noticed it at 1.5.2)
If I have either the Palette 2 or Canvas plugin enabled on the latest OctoPrint (1.5.2) then no plugins will be listed in the Software Update section. Disabling both plugins will then cause all enabled plugins to be listed in Software Update and be updatable again.