Soil that has not been prepared like if it is still in sod, or just too dry, we get that here, can be tough on a walk behind tiller. Unless you are in sandy or easily worked soil, starting from scratch in either sod or dry/hard soil is tough to deal with. I prefer to moldboard plow mine and another smaller patch I cannot plow, I literally had to soak the area to help soften it up this year it was so dry, I did not get to tilling it last fall, was sick, add some rocks under foot, can be a bumpy ride! This patch I have worked for years now, but started out as thin top soil over clay/gravel fill that was in grass, part of the lawn, had to take it in layers, with the depth setting and not too much throttle on the troy bilt horse. Both of those, throttle and depth need to be adjusted right to work these soils, you go full depth at full throttle, in hard or sod bound soil, forget it, thats why they have those guards on them, so when it takes off and turns over you don't knock off the carb LOL ! You likely know about this, but I always mention the above, I find that the horse with a K161 Kohler, 7hp, is the perfect engine in a horse, I can get the right torque value and tine speed for starting off in sod or working soil when its dry, both which I hate even considering doing.
On the other side of this, and the troy bilt tiller was called a power composter, you work one of these in deep organic matter/soils, its real easy, we have a patch that was a pile of manure, but now deep like in several feet of it, that I started planting a garden on, at our other place, tiller with worn out tines, a TB horse with the 8HP magnum kohler, does an excellent job in no time.
The bottom line is creating a garden area with easily worked soils, there is no way around that with a walk behind rear tine tiller. It makes the task so much easier, even if you have to excavate out the unsuitable material and replace with thick organic matter and the right amount of other desirable soils.
I have basket ball and larger size rocks in one patch, then it gets hard when dry, clay/loam/silt/topsoil, not bad, but after plowing first, I got most of the rocks up, soil turned, loose enough to got full depth and just have to watch out for more rocks, same patch I did start from scratch after 10 years in sod, and it was progressively a pain in the @ss, just given the soil type, it had been worked for many years before it sat idle with a TB horse, and was always not so easy and wears tines out, finally got a plow and plowed in the fall, then spring, then use the tiller, so much easier.
I do have a TB junior, never tried the one in the middle, but it would seem both are too light too fool with in unworked soils, I have a friend who worked in Gardenway's proving grounds, he'd tell you about the same thing. The only other suggestion is to play with depth and throttle, that is the only way I've been successful on unworked soils, or you need the heavier/larger tiller, like the horse. I got the junior for resale or to try for cultivating, no way it would suffice for primary tillage here.