Hi Shelby,

The amount of product (intensity of the bands) mostly depends on the amount of template (i.e.DNA) you put into the reaction. Theorectically the more template you put in the more product you should amplify. 

Other factors could be such things as annealing temperature of the primers, Mg+2 concentration in the buffer or some sort of inhibitor present in your template that prevents efficient amplification. Hope this is helpful.

Or perhaps, some DNA samples are of not good quality, are degraded and present troubles in the amplification