What will cause my EL-34 filaments to glow differently.

It sounds like the bias is off and the plates glowing. Check the bias.

Swap them and see if the same tube is brighter. If so, it is most likely that the filament (which is what glows) is just sticking up and out a little more from the cathode sleeve it is slid into, which is just a manufacturing variation, is common, and not a single thing to worry about.

it is possible for different tubes, esp. tubes from different MFGs or production lots to have subtle differences. if the difference can be compared like a 15watt bulb vs a 300 watt bulb, check voltage, and if possible current draw. if the ghetter has turned milky in the "brighter" tube, the tube may have lost its Vacuum = bad tube. Otherwise need better description of problem.