Decrease the percentage of agarose gel, probably you are using a higher percentage of agarose gel for a bigger DNA size. What is the size of DNA and what percentage of gel you are using?
Hi, The probable reason being improper sampling, the presence of DNA binding proteins in the sample.such as ligase, phosphatase or restriction enzymes may alter DnA migration in gel.
Repurify the DNA carefully by re-extraction with phenol-chloroform/chloroform-IAA so that high molecular weight protein junk will be removed and you get pure DNA. By the way may I know which DNA plant or animal. For checking DNA 0.6 or 0.8% agarose gel is sufficient. Good Luck!!