What factors could lower the voltage of a galvanic cell
Although it seems simple to make a galvanic cell ( i.e. a battery) it is really fairly complex. 


The theoretical voltage is given by the Nernst equation.
The actual measured voltage also depends on the current. The equation
$V=iR$ still applies. So various restance sources in the cell cause voltage drops. 


  
    
    The current must be very low so a high impedance voltmeter must be    used. (Not really a problem these days with modern semiconductor
    voltmeters as opposed to galvanometers.)
    The salt bridge offers resistance, so it has a voltage drop.
    Impure metals for the anode or cathode can cause a voltage change.
    Surface coating can cause a voltage drop.
    The acid or base concentration can cause a voltage change.
    The ionic strength of the solution can change the voltage.