This advice is from Google Play:
If you receive a “Download unsuccessful” message when attempting to download an app, try the following troubleshooting steps to resolve the issue:
For paid applications, confirm your credit card information is up to date by signing in to your Google Wallet account.
Clear the cache of the Google Play Store app(visit Settings > Applications > Clear cache).
Restart your device, then try downloading the app again.
Wait 10 minutes, and try your download again.
Confirm there is connectivity to either a wireless or cellular network on your device, and contact your service provider if you aren’t connected.
Ensure there’s enough space on your device for the app to install.
If none of the above steps work, try remounting your SD card.
Visit Menu > Settings > Storage > Unmount SD card; then remount the SD card from the same menu by selecting Remount SD card, or restart the device if the remount option isn’t available.