What could cause a 'technical failure' in a DNA test?

Question

DNA was purified from the tissue [someone] brought me, and from human cells (positive control), and the DNA from the sample of the control human cells was reacted with three sets of primers designed to amplify (make copies of) human or chimpanzee mitochondrial DNA (mtDNA). Two of the three primer pairs worked - produced DNA segments - of the size predicted for human/chimp DNA. The third primer pair failed, even with the positive control (human) sample, so the failure was only a technical one.

Answer 1

Sedimentation . Subsequent aspiration of supernatant low in tissue sample in one of the the three pairs.

Answer 2

The thermocycler conditions may not be optimal for that primer pair.  You may have to tweak the temps and hold times.  Also make sure that the primers have similar GC content.  That will be especially important if this is a multiplex.