Our engineers have advised that the Macbook BIOS may not be recognising the 100W charger so even though the Flash is outputting 100W, the Macbook is only detecting 60W.
Flash isn't a wall charger. It's a powerbank which has a battery and is hence detected differently due to different PCB vs a wall charger.
Please perform the following test to confirm this:
Drain your Macbook to 0% and charge up Flash to 100%. Then with your Macbook switched off, charge up your Macbook using Flash. A full charge of Flash should charge up your Macbook around 52%. To see where we obtained this number, please check out other guide here.