When using artus® PCR Kits, why is PCR sometimes inhibited?
FAQ ID -1401

An amplification failure of the internal control (IC) is indicative for a PCR inhibition which may be caused by substances in the eluted DNA/RNA material.

Especially when handling sample materials potentially containing a large number of inhibitors, such as stool or urine, complete removal of these agents is crucial for optimal PCR performance. Likewise, the quantitative elimination of ethanol prior to elution of the DNA/RNA is essential to prevent PCR inhibition. Repeated inhibition may be observed when metal swabs are used as sample materials (for example, in combination with the artus® C. trachomatis LC PCR Kits) since heavy metal ions may come off the swabs. These ions could strongly impair the Taq polymerase activity resulting in failure of the PCR.

For a more detailed description of different PCR inhibitors please refer to Wilson, I. (1997) Inhibition and facilitation of nucleic acid amplification. Applied and Environmental Microbiology 63 (10), 3741-3751. Please note that an absent IC signal could also be caused by an inefficient extraction procedure (provided that the IC was added to the extraction).