[L473ncy]

Could it be the loading nozzle is slightly off or something?

The not feeding unless upsidedown sounds like it might have to do with the spring in the mag not having enough tension to push the BB into the chamber. That would however be odd if it's a BNIB mag.

Have you tried to maybe shim the magwell or the drum mag so that it gets pushed the right way and will feed? I know that with some guns you'll have to file away at your mag or magwell and sometimes add material (ie. layer of masking tape to shim it forward/backward).