Why the change of the position of the salt bridge or metal piece leads to different voltage value?

I am doing an experiment where I use salt bridge to create galvanic cell. The goal of my experiment is to find if there is some correlation between the amount of solvent being used and the voltage being produced which means the accuracy of voltage is very important.


I am using zinc and copper metal pieces, zinc sulfate, and copper(II) sulfate with sodium chloride for salt bridge:


[![Galvanic (Daniel) cell](https://i.stack.imgur.com/WYe4Y.jpg)](https://i.stack.imgur.com/WYe4Y.jpg)


The experiment works, but the problem appears that when I modify the position of either the metal piece or the salt bridge, the voltage value will change, sometimes I will change a lot (e.g. $\pu{0.3 V}$ to $\pu{0.42 V}).$


Does the change of the position of the salt bridge or metal piece leads to different voltage value in galvanic cell?


If not, could it be the reason that my zinc sulfate powder does not fully dissolve in water? So my metal piece could touch the area where there are more Zinc powder which causes it to be more electron transfers happens. It is very hard to get it fully dissolved in distilled water. Any suggestions for improving solubility in water?


I am really looking for way to reduce the uncertainty of my experiment, but the different voltage measured by only changes the salt bridge or metal pieces makes me very concerned.


