There may be no apparent "progress bar" in the App Store. It may be a bug associated with this particular download.
To determine if it is actually downloading use Launchpad. You should see a fairly obvious new icon with an associated progress bar. Make sure you check every Launchpad page by swiping left / right.
If it really is not downloading read Mac App Store: How to resume interrupted downloads
- Open the Mac App Store.
- From the Store menu, choose Sign In.
- Enter your Apple ID and password.
- Click Purchases to get to your Purchases Page.
- Click the Resume button next to the app name.