As I'm sure a number of people have done, I frequently charge my MiFi Box off a battery pack. Today I encountered a strange situation which I have no explanation for.

I plugged in my battery pack to charge my box, the bank just turned itself off after being plugged in (I.e. all the lights went out)

Naturally, my first thought was dodgy cable, tried a different one, same issue. Then thought, battery bank is playing up, so I tried a different one. Same issue. The MiFi Box won't charge of battery packs it has charged off for months.

Then thought, maybe the connector inside the MiFi Box itself has gone, so tried charging it off the mains. Charged without any drama... Has got any idea why it won't charge off a battery pack, and any potential fixes? 

Thanks in advance for any advice