Power banks normally output approximately 60%-70% of their capacity to the connected devices, and this is due to the fact approximately 30%-40% of the battery capacity is consumed by the power bank itself to actually generate the charge to the device it is trying to charge. Please note these efficiency issues affect all electronics devices.

For example, a fully charged Anker 525 power bank (20,000mAh) can generally provide 4.5-5 full charges to an iPhone 12 that has a built-in 2,815mAh battery. 

If your power bank does not function properly, please provide the following information:

• The make and model of your phone, such as Apple iPhone 12.
• How many charges can the power bank provide your device when the battery itself is fully charged and your device is in power-saving mode (not being used).
• Try using another charging cable.
• Verify if the power bank can charge other phones/devices without issues.