Wiring Help - Fire HD 8 Wall Mounted - Not Charging despite showing charging indicator

Finally got around to wall mounting my Fire HD 8 as an ActionTiles controller; and am leveraging the existing legacy alarm control panel wiring for power alongside the Konnected setup.

I've placed a 12v to 5v 3a power converter in the alarm box in the basement to provide power the tablet, and see a steady 1.8amp and 5.1 volts at the tablet mircro-usb connector when testing with my multimeter.

Despite this, the tablet will not maintain a charge, nor recharge. It shows the charging icon, but the battery quickly loses power (same as unplugged - not "slowed discharging" as if its drawing more than the input).

Looking for a sanity check here with anyone who's wired up their own custom USB cables. Its pretty simple; red to positive, black to negative; check voltages and mA, and should be good to go right?

Amps: https://imgur.com/t2Ig14T Voltage: https://imgur.com/etC5KmY

Needed to place the power converter in the project box behind the tablet - the wiring run from the alarm box had too much voltage drop under load and the tablet was only pulling 250mA. By placing the converter in the project box, the wiring run was 12v, which allowed for voltage drop before being converted to 5v. 

Thanks for the tip, it probably prevented me from making the same mistake!