There are several reasons behind this occurrence. For one, moisture and bread type might affect the toasting procedure. Since the moisture content found in certain bread types such as white bread takes up a lot of heat, your toaster might toast just one side.  Another reason could be the temperature of the toaster. The spacing of the wires that induce the heat to the bread may result in an uneven distribution of heat to different parts of the slot. The toast space might be another possible reason behind this occurrence. Just like smaller slots can't accommodate larger slices of bread, large slots cannot hold the smaller bread. In such scenarios, inefficient heating may result in a one-sided toast.