Effect of side reactions on galvanic cell using iron rod as anode and sodium chloride as salt bridge
Since you state, &quot;We measured a voltage way below the expected value and it was dropping quite fast,&quot; it indicates a few things that could lead to those observations.

How did you measure voltage? If you used a low impedance voltmeter, it would show much lower voltage because the impedance of the meter is low compared to that of the cell. Voltage is distributed in proportion to the impedance (here, the DC resistance) of the meter and the cell. For example a 1 volt cell with internal resistance of 20 kilohms, measured by a voltmeter with resistance of 20 kilohms, would show as only 0.5 VDC, because half the voltage is dropped inside the cell.
As a cell delivers current, the surface of the electrodes change. Corrosion, surface deposits and bubbles can cause current to drop. While the cell is being measured, observe the electrodes closely. Look for any change in appearance that progresses during measurement.
How well were the two half-cells isolated? Did the sponge effectively block liquid (as opposed to ion) flow between them? You can test that by putting some dye, such as food coloring or bromothymol blue, on one side and seeing if it migrates to the other. Flow might indicate the cell was &quot;short circuited&quot;.