I tried amplifying some plant DNA I extracted with 5 types of primers. In some DNA samples, I observe electrophoresis gel bands just with one PCR procedure. But some samples do not show gel bands after one PCR but shows gel bands when I do a PCR, (with the vial that I already did the first PCR) for the second time. This happens with all the primers that I am using. Does anyone know why something like this could happen?