When the download does not start, the following may be the cause.
- The download may be restricted depending on the remaining battery. Please plug your device into the power to continue downloading.
- The download is restricted when using "Low Power Mode." Please turn off the "Low Power Mode" and try downloading again.
The download may be restricted when many apps are running at the same time. Please close other apps that are running and try downloading again.
- It may take several minutes for the connection to the carrier's network to switch after turning off [Available on Wi-Fi Only] in the "Download Settings". If the download does not start, please try it later.
If these above does not seem to be your case, please refer to this article: Troubleshooting iPhone/iPad/iPod touch.