It is often the case that when the primers do nothing then they form a primer dimer but when the sample amplifies then there is no dimer. perhaps in your example one primer is annealing and extending while the other is not so there is no product but the presence of the sample dna is removing some primer from free solution so lessening the chance of a dimer forming. It could also be that PD needs the enzyme to work which need magnesium but the dna in the sample tubes has inhibitors that removes Mg from solution so the enzyme activity is lowered so less dimer. If you are looking for baceria in tissue there will be very little bacterial dna in your sample and it may take a hemi nested or fully nested second round of pcr to get a good amplified product to show on the gel.

It is often the case that when the primers do nothing then they form a primer dimer but when the sample amplifies then there is no dimer. perhaps in your example one primer is annealing and extending while the other is not so there is no product but the presence of the sample dna is removing some primer from free solution so lessening the chance of a dimer forming. It could also be that PD needs the enzyme to work which need magnesium but the dna in the sample tubes has inhibitors that removes Mg from solution so the enzyme activity is lowered so less dimer. If you are looking for baceria in tissue there will be very little bacterial dna in your sample and it may take a hemi nested or fully nested second round of pcr to get a good amplified product to show on the gel.

What are you using as a negative control solution? Is there a salt difference between the sample solution and the NC solution? eg TE or other buffers may change the free Mg ....what is the expected product size? How many repeats have you done? do you always get the NC band?

What I guess, your samples had comparatively higher concentration of DNA (may not be the bacterial but of human DNA) than the negative control (no DNA) at all. Now, primer dimers generally form when they bind with each other, which is probable in case of NC as the primers don't have chance to bind with any DNA. Whereas, in your samples the primer can also attach with the human DNA as well (if some amount of complementary bases found), though it's not a perfect pairing to extend the complementary strand, still it highly reduces the chance of forming primer dimers. Therefore, you are not getting any amplification/dimers in your samples!