Complete STR results are obtained from the far left stain and the upper right stain, but not from the center of the large stain. Why?

Consider the following photograph. This shirt was recovered from a murder suspect’s basement. It has been packaged in paper by the lead investigator and is in this condition when received at the laboratory. The DNA analyst chooses 3 samples, as delineated by the arrows: one from the stain to the far left, one from the center of the large stain and one from a droplet on the upper right. Complete STR results are obtained from the far left stain and the upper right stain, but not from the center of the large stain. Why?