smear in pcr product, but of same size as that of positive product

[uru]

I have a bizarre problem while doing PCR for transgenic plants. I'm getting distinct band in positive control=plasmid DNA, but i'm getting smear in case of my plant genomic DNA; "and the smear is forming in the exact size range as that of positive control". That is, smear is not from the well. How is this happening, and what are the trouble shoots? Plzzzzz reply ASAP, i'm in dire need.

[gautama]

Thanks for your replies rhabdo,
I've been busy these couples of week, so I didn't response to your answers. Yes, I am looking at the soil microbiota as a whole rather than isolating specific bacterial species. In my DGGE profile the intensities of each band is about the same at each lane
(seen at the same row), that's what make my head aches. I was hoping to see a different profile lane to lane, but it seems they were all look the same! But from one band to another in the same lane/column have different intensities. I used 338F-GC and 518R. Any ideas how can I avoid two bacterial species from stopping at the same point in my DGGE gel beside using different primers?
Thanks for your kind answer...