i know this is going against popular opinion but try running ur gel at a fixed current instead of voltage. a good try would be 15 mA to 20 mA depending on the size of gel. if u are able to run it at cold temperature, i.e. cold running buffer in a cold room or fridge.
for my case this greatly increased band intensity, especially the smaller ones that migrates close to the dye which usually diffuses.
there are logical reasons for this, i think u can look it up in textbooks.