Why does high concentration of DNA templates obstruct PCR?

I have extracted Bacterial DNA from sea water, and I get around 165 µg/ml for the PCR. I used Promega PCR mixture, they suggested to use 50µg/ml of DNA template for the PCR. I tried to use 6x DNA template (2µl of DNA template) & I have no band. When I decreased my DNA template concentration (0.5 µl, approx 50-80ng) I got a band.