An agarose gel was run on a PCR product, but no bands were detected. You review the PCR reaction protocolwhich included a thermostable polymerase, template DNA, complementary primers to the template DNA, and dNTPs. The mixture was heated to 95 C followed by a cooling cycle to 50 C and repeated 50 times. In a control PCR reaction on a different template DNA with complementary primers, a PCR product was observed so the PCR machine is working correctly. What could be an explanation why you do not see a PCR product ?