I isolated microbial DNA from marine water samples taken at various sites using the Qiagen Power Water DNA Isolation Kit, including a modified enzymatic lysis step. I encountered a situation where the 16s PCR amplification worked perfectly with the DNA from the same sample, but using the same sample, PCR did not amplify. I tried using both short and long-read primers but still faced issues. I'm perplexed as to why the DNA from the same sample did not amplify. Any suggestions on what might be causing this issue would be greatly appreciated.