If you have a device that can be charged at the maximum 100W speed and you only see charging at 60W or below.
1. You are using the correct USB-C cable that can charge at 100W.
Power Delivery (PD) is a specification for handling higher power and allows a wide range of devices to charge quickly over a USB connection.
It operates by facilitating a conversation between two devices to negotiate a power contract so they can determine how much power can be pulled from the charger. Power Delivery starts at the 5V setting and is configurable up to 20V.
Using a standard USB-C cable, it can handle up to 60W, and will go up to 100W using a designated EMCA cable.
2. There are not any other devices connected to it
To see how power is distributed when multiple devices are plugged in, see this article: https://hypershop.zendesk.com/hc/en-us/articles/360039110012-How-is-the-power-distributed-
3. There are no Apple Lightning cables plugged into HyperJuice GaN charger
To see how Apple Lightning cables affect the GaN charger, go to the "Note about Cables" section in this article: https://hypershop.zendesk.com/hc/en-us/articles/360040736911-Having-issues-with-the-GaN-charger-Check-here-