" Note that for any download attempts from Google Play on the web, you must have strong connectivity on your phone or tablet at the time you push the download from play.google.com." 

If your carrier doesn not provide at least 3G connectivity it is often better to use wifi for an update so you get all the file not just part of it and to avoid a corrupted file. As far as I can tell Google does not use a checksum to verify a download so retries may be necessary if the app won't launch correctly.