The most common reason for this is that the pump was paired with the app without first doing a wired upload to t:connect using a computer. Depending on how long it has been since the last time a pump was uploaded to t:connect, it can take anywhere from a few minutes up to 3 days to complete your first upload via the mobile app. Since it uploads your oldest data first, it may appear as though the app is not uploading. You can either wait until the app is finished uploading, or connect your pump to a computer and upload via USB.
It's also important to confirm that the app is open, and that your phone has either a Wi-Fi or mobile data connection. The app has to be running in order for your data to upload.