When plugging two USB-C devices that support both sinking and sourcing power together they will pick a role at random. Perhaps some devices will prefer to source than sink, or vice versa, so maybe it isn't so random. There's a short memory on these to allow swapping roles by unplugging and reconnecting. This gives the impression to some people that a USB-C to USB-C cable has a preferred direction of power flow and by flipping the cable they are redirecting the flow. That is not the case, assuming the cable is built to spec, the disconnect and reconnect cycle is what is reversing the flow.
While a laptop can sink something like 100 watts of power they can typically source only 7.5 or 15 watts, that would be 5 volts at 1.5 or 3 amps. If your power bank is capable of charging from 5 volts then you should be able to charge it from your laptop, but it might not be as fast as you like.
One way to force the direction of power from USB-C to USB-C is with the use of a USB-C/male to USB-A/male cable and a USB-C/male to USB-A/female adapter. With the USB-A connection in the middle power can only flow from the device at the adapter end into the device at the cable end. The power flow will be limited to that allowed by the USB-A port but the laptop will likely have the same power limit out from the USB-C port. This might be helpful because some people find that with a USB-C to USB-C cable a power bank will charge from the laptop, reset the connection, and then discharge into the laptop. That means finding a dead power bank in the morning.
I've found that laptops will often choose to draw power from whichever attached power supply reports being able to supply the most power. If your laptop power supply is rated for 65 watts and the power bank is rated for 80 watts then the laptop might prefer to draw power from the power bank if using a USB-C to USB-C cable. Putting a USB-A adapter in the USB-C port on the laptop, then connecting the power bank to the adapter by a USB-A to USB-C cable (male on both ends) then power can only flow out of the laptop into the power bank.
This means carrying some extra cables but it adds some convenience in not having to "babysit" your power bank as much. Use a USB-C to USB-C cable to connect the laptop to the power bank when you want power from the power bank to the laptop. When you want power from the laptop to the power bank then use the USB-A/female adapter on the laptop and the USB-A/male cable on the power bank to connect them.
It's quite possible that the power bank will refuse to charge from the laptop because it is supplying only 5 volts and it wants to see 9, 15, or 20 volts. I have not seen any laptop that will supply more than 5 volts from it's USB-C ports so it is unlikely your laptop is any different.
There may be other solutions to your problem but to offer them I'd like to know more about the power bank so I'm not speculating too much.
