FAQ: What are the main causes for blunting reaction failure using DNA Polymerase I, Large (Klenow) Fragment?

The following conditions can cause the exonuclease to overwhelm the polymerase activity causing recessed instead of blunt ends:
*Adding too much enzyme
*Incubating for too long
*Incubating at temperatures greater than 25°C
*Failure to add nucleotides or the nucleotide level is too low
*Heat inactivating without EDTA