As T-Mar and rhm suggest, the fork steerer is so short on that small frame the bottom of the stem is probably running into the butt taper before clearing the minimum insertion line. Here's a typical steerer tube from Nova showing the length of the butt (50mm) and the length of the butt taper (30mm). That's a total of 80mm from the bottom of the steerer tube (even with the bottom of the fork crown) and the beginning of the taper. On a very short head tube frame, some of the steerer would typically be cut off at the butt end to provide room for the quill stem to be inserted at least to the minimum insertion line.