Ariens Deluxe 24 zero turn drive issue

This nice machine is 3 yrs old and probably doesn't have more than 5 hours on it.

While the zero turn feature is nice, I've encountered problems with getting both wheels to drive. It seems if it encounters any resistance, the right wheel will drive and the left wheel does nothing. I end pushing the thing will a ton of effort just to get it move forward. I've been through the manual and made all the adjustments. If it's just dry powdery snow - no problem. With anything else, it's a struggle.

Any ideas ?