The adjustment of L103 is critical for stereo operation.
If the incoming station, the tuner front end or the IF gain is weak then the circuit mostly composed of H109/L103/H118/H119 won't be able to turn H302 off to enable the multiplex decoder, H301 to decode stereo (If H301 pin 8 is at or near zero volts, then the decoder is forced to mono operation).
In the last 2215B I rebuilt, L103 was bad and I had to get one by buying a donor board from the auction site.
To adjust L103, tune in a strong stereo station and monitor J114 with a voltmeter. Use the proper alignment tool to adjust L103 for maximum positive voltage, which should turn H302 off.
Tom