What happens when you view these apps in the store? Do the buttons at the bottom indicate they are installed?
I've come across a couple of apps that I wanted but the store listing indicated that it was not available for my phone. Do you get any of those messages (they usually appear toward the middle, just below the app avatar/icon)?
I noticed that the 620 only has 512 MB RAM. Checking Sleeve Music, they don't indicate how much RAM is required, but one of the reviewers stated that it didn't work on either a 520 or an 820.
The store listing for the Nokia Camera states that it works best on phones running the Cyan update, but that works with limited functionality on phones with the Amber update or later. You can see if you're running Amber or Black in Settings > Extras+Info. The store listing also does not state whether 1 GB of RAM is required.