Absence of bands in some lanes of gel electrophoresis - What are the reasons? (Sep/16/2006 )

I did a PCR reaction followed by agarose gel electrophoresis using several samples of hair follicle DNA... The results i got were very nice for some samples but for some samples there were no bands at all in the gel.. All samples were run in the same gel so i don't think it could be due to problems with the reagents... What are the reasons that might cause this?
I was thinking it might be due to handling problems... Besides maybe i vortex too vigorously is there any other steps that i should take note of?

Could it be that you didn't have any product in some of the lanes?

You can`t do anything wrong after a PCR reaction. Keep it on ice. Load the gel, you´re done. No signal in lanes, no product, it´s simple as that.
Check your dna purity for each sample before setting up the pcr. maybe some of the samples just suck and have a low purity.
Can hardly imagine that vortexing is the problem...mastermix is all the same for your samples. Don´t think this is the problem.
Maybe some of your samples might need a nested PCR.

Good luck