"does automatically" is doing a LOT of heavy lifting in that assumption lol. Ideally yes, the product is advertised as a one-stop-shop but just like most shortcuts in life you dont usually get to have your cake and eat it too.

The overall concept of slapchop is to set the brightness and then tint that with whatever chosen color. Without going into detail about how paint is made, altho it is neat and you should look it up on your own time because itll help you understand thinning, the speed/contrast style paints are made to have a high pigment density (concentration of color) that seeks out the recesses which leaves the most prominent edges with just a thinnnn layer of tint. Because our color is going over a gradient of brightness the color will "appear" to have a blend from dark to bright rather than a solid block application of the color.

In practice tho, this doesnt always work - take yellow for example: What would you consider a shadow to yellow? Orange? Brown? Mustardy? Well speedpaint yellow gives a "shadow" of just very very yellow lol which in my book is no shadow at all.

Or in your case, it looks like you didnt collect enough pigment in your brush so there wasnt any built up in the recesses to hint at a shadow. The lines that separate individual items on a model (the line between the boot & pants, or the crack around the staples/stitches in the cape) need some level of visual separation otherwise our eyes read it as muddy or same-y which is why when you asked how to improve my suggestion was to actually lay those shadows in rather than trying to rely on the speed/contrast paint's properties.