A USB charger on a dynohub rarely puts out more than about 500 milliamps (half an amp) while riding at typical touring speeds.

If your powerbank is a big one with many thousands of milliamps of capacity, it could take many hours to put a noticable amount into it.

Occasionally but not often, plugging in a device with a micro USB to charge it can cause it to discharge. I have generally blamed the micro USB plug for this. It is quite rare, but has happened to me.

I am an engineer, although not an electrical or electronic engineer, still I learned some basics of power systems in school. I often use an ammeter when diagnosing such things. You can buy a really cheap one on Ebay, shipped from China (might take a month for shipping), do a Ebay search for "usb charger doctor". Then do a sort to put the lowest price (with shipping) at the top, then select buy it now. Get the gray one, not blue, the gray is impossible to read on a sunny day, but almost readable on overcast days, the blue one is much worse. That will tell you what is coming out of the USB port while you are charging something, both volts and milliamps.

Some devices, especially Apple devices can be very picky on the power supply. If I recall correctly, The Plug has a cache battery, so that usually helps a lot. But some devices pretty much need to be charged from a power bank, then charge the power bank from the USB charger on the bike.

Good luck. I am leaving for a 5 day trip (trying out a new bike), so if any questions I won't respond for a while.