Different tillers have different depths that they can till to. Usually the larger the tiller, the deeper it can till.
On most tillers that I have seen (and I have a Land Pride RTR0550 tiller) there are two skid shoes on either side of the tiller. To get the maximum depth, lift the tiller with the 3 pt ht and unbolt the fixed position of the shoes, and then push the shoes up to the maximum height (right up against the tiller) and then rebolt them in. With the shoes up against the tiller, the tiller will sit as low as possible in the ground.
Even so, on my tiller, one pass often is not enough to reach the maximum tiller depth for the skid shoe setting. I often have to make 2 or 3 passes of the same ground in order to 'reach' down to where the float pan on the back of the tiller is amost parallel to the ground.