I have issues every so often with the Marketplace, especially when trying to update apps. I usually end up having to do some of the following to get it to work:
- Reboot the phone
- Uninstall the app and reinstall
- See above, and try that a couple times
- Download it using the Zune desktop software and then sync my phone
It's pretty annoying but until the March update comes to the Focus we may have to live with this.