I have my Bambu A1 set up in a very tight space, so I made use of the AMS Micro by emics to mount my spools vertically.

The printer is able to reliably load and unload filament from all four spools, but during printing (especially slot 4, the topmost spool), particularly after retractions, the extruder will be unable to re-insert the filament and will throw an "extruder is not extruding normally" error. I can manually push the filament in until it's grabbed by the extruder after which the print will continue normally until the next retraction.

The spools spin freely on the stock AMS hubs and the PTFE tubes are short and not worn. Any advice would be most welcome

Yeah, just land that service ground wherever. It all ends up at the same place anyway, right?

Not one I've seen before. Trust no one, not even your helpers