The aux tanks (10 gallon each in the box-section of the wing, just outboard of each main gear) should feed equally to the engine, and the return fuel from both should return to the left main. If you have no return fuel to the left:
Is it returning to the right? Or is there no return, period?
If returning to the right, you have a defective fuel selector, likely in concert with a plugged fuel return to the left.
If no return period, it must be running awful rich, likely in concert with a plugged return to the left.
The plugging, if only affecting the aux and not when feeding the left main, it is in the fuel selector or the short line that connects return fuel from the selector to a 'tee' in the left sidewall just forward of the selector.
I would recommend this gets addressed before further flight. If there is something amiss with the fuel delivery system, we don't want to read about you in an NTSB report.
Station one person inside the cabin and one at each main filler port; fuel is returned to the main tanks at the outboard end, high in the cell; when it is returning, there are noticeable waves and bubbles that appear to flow. Select left main tank, mixture lean, throttle open, and turn on aux fuel pump; there should be return present at left main only. Repeat for right main and there should be return to right main only. Repeat for aux tanks, and again it should return left main only.
MIke, Any answers as yet? I had a similar issue with my 1960 Deb. Left aux drained fine, right (passenger side) aux tank would not drain. My A&P assessed the check valves and the fuel lines. No joy. Sold to another pilot. Was having his A&P evaluate. I will text him to see if any resolution. Good luck.