You might try resetting all of your iPhone's settings to rule out invalid or inaccurate settings as a possible reason. This will clear out all of your device's settings and return them to their original settings. Any faults should be automatically removed as well. Follow these steps to give it a shot:

1. From the Home screen, choose Settings.
2. Select General.
3. Reset may be found by scrolling down and tapping it.
4. Choose Reset all settings from the drop-down menu.
5. Alternatively, you may reset your network settings and address Gmail app difficulties caused by network faults and glitches by performing a network settings reset.
6. Continue by entering your device's passcode.
7. Then validate that the settings have been reset.