Riding lawn mower transmissions are normally a one wheel drive at a time. You may notice it especially on hills where one tire slips on the uphill side.
On many riding lawn mowers, this is a perfectly normal situation. There is a sort of limited slip built into them and slippage like this is common.
If your riding lawn mower is supposed to have both wheels turning, then you may have a transmission problem or one of the wheel keys to holds the wheel to the axle may not have been installed after recent work.
One way to tell if your transmission is working properly is to put the rear up on jacks and then spin one back tire. The other tire should rotate in the opposite direction on a normal lawn mower. They should both turn in the same direction on a mower where both wheels engage.
Check the wheel keys. They are just a square piece of steel that slides into the axle and the wheel itself.
If you cannot find the source of the problem, visit one of our Locations for a riding lawn mower shop near you.