How can I balance the tyre tread on the rim?
First, deflate all of the air from the wheel. Once the air is removed, reposition the tyre tread so that it is even all the way around the rim. Once in place, reinflate the innertube (while holding the tyre tread) and it should allow the tread to stay in the correct location to allow the stroller to roll smoothly. It may take a few tries, but this will almost always fix the issue.