Hello there, I've had the same issue all day, but I *think* I've fixed it, I'll be more sure in a few hours when the downloads actually had time to progress...
- Pause the download by clicking on the app
- Click and hold the app icon until you get the 'wobblng' effect and the little crosses come up
- Click the cross and say yes to deleting the app
- Re launch the app store and start over.
You can check that the app is downloading by using activity monitor (->Utilities. Select the "Network" tab in the app) to see if there's a continued spike in your "data recived". This is best seen whilst not downloading anything else so you have a 'rest' level, then begining the download. The jump in numbers and graphs will make it pretty clear.
NOTE: The app hasn't re-appeared in my purchased list on the app store. I deleted another app off that list and re-downloaded it for a comparison and that didn't come back either, but still downloaded successfully, so that's just probably just another one of the app stores delightful quirks and not nessiserily tied to the issue. But don't take my word for it =P
That's what I've done and so far so good 🙂