Same frustrating problem for me with 12054 operating/unloading tracks----have 3 of them and with all three the uncouple works perfectly but the unload is something else!!!!
Did you have occasion to try a DPDT replacement switch and beefier wire (as suggested earlier) in place of what I call the brass-leaf contraption in the stock controller? In addition to the DPDT examples provided earlier, the up-down control button for power-windows (momentary center-off DPDT) is widely available and quite inexpensive:
If you are not familiar/comfortable with switch terminology, don't hesitate to ask here before ordering! Terminology like DPDT, center-off, momentary, (ON)-OFF-(ON), and so on apply to what you want - in other words, alphabet soup!