How to get Steam to download a DLC?

I've been in game. It hasn't downloaded. It isn't in my list of available DLCs either in game or by checking the list when right clicking the game title in the Steam library.

I've tried a few times. The DLC list in game is showing a ! which I assume is to let me know it is released, but it isn't showing up.

This is by far the best tip here. Doing this should resolve all issues.

Right, I'll try that, but I don't see how it will force a download of the DLC. I thought it just checks for corrupted files and the like?

And haven't restarted Steam, normally downloads will just start whenever they show up, so never thought of restarting. I'll do that after the other tip.

It didn't, which was the problem. I wanted to know how to force the download.