Gel Electrophoresis

You visualize the PCR product from the eight mice and the positive and negative controls, on a 1% agarose gel, as shown below and in the video. You also use a DNA ladder, with a molecular weight ranging from 100-600 bp.

You are confused that there are no bands showing up on the gel and ask Dr. Demetriou for his help. Dr. Demetriou explains that there could be a number of reasons to explain the results but first asks for your opinion. 

What is the most likely explanation for these results?