Why are my Buck Booster and Power Bank not playing together?

[sirdavy]

You do realise that stepping up the voltage reduces the available current pro-rata?
2.4A @ 5V = 0.6A @ 18V (assuming close to 100% efficiency).

Reduce the boost voltage until it works.

POWER BANK>CC CV>BUCK BOOST>AMP is the one I'd go for.

Yes, I was thinking something has to give when stepping up. But - and I'm not sure if I'm making a mistake with the DMM or misremembering - I think I measured the out put of the buck booster plug into the power bank on its own and it read 18v/2.4a. I'll check again.