First of all be sure to have the latest version, higher than 3.31.
Open the Settings tab of the iPhone app. At the bottom, on the left, you will see "Send Logs". If a number is present, you will see the count of tracks still to be uploaded.
If you open the Diary, you will see some tracks with a UP ARROW aside from the date value (in green).
Furthermore, the upload and the download from the Cloud works only with the app open. You will see an orange progress bar in the Diary tab for the download, and an orange progress bar in the Record tab for the upload.
Read also this article for more info: Cloud Sync with iPad & Backup of the Diary