Powerbanks not charging Apple Watch reliably

I purchased a portable power bank specific to the Apple Watch series 4 and what’s odd is that it starts charging for a few seconds and then stops, then starts and just goes around in a cycle every 5 seconds or so. I sent that back and purchased a different one from another retailer and it’s the same issue.

finally I purchased a power bank not specific to the Apple Watch and it charges other devices without issue but it’s the same story with my watch. If I connect the watch USB cable to the wall outlet it works absolutely fine.

i doubt the watch is faulty as it works from a wall outlet via any USB charger - the problem is only with powerbanks And I’ve even purchased MFI certified ones.

any ideas what I’m missing here? All I want is to be able to charge my watch on the go from a portable power supply. It can’t be that difficult, can it?