The issue is probably the adapter. There is no standard for how they're wired, and some are wired as mono, others as stereo. Note the other hassle with plugs and jacks is that some are wired as headset/mic combos, and others are wired as stereo headphones. You (obviously) need to use one that is wired as a headphone/mic combo.
If you had a headset with a 3.5mm plug, it wouldn't matter if it was 3-conductor or 4-conductor, since the R and L channels would just be electrically combined with a 3-conductor female jack. Look at the position of the rings on the connectors and you'll see what I mean.