Voltage on circuit input different than on power supply
As specified in comments, it is very likely that you are experiencing excessive loss in your cables.  If you are using very long wire or wire of a too small gage, you are actually getting such an equivalent circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Where the vertical resistors are the "computers" modeled as loads and the horizontal resistor are losses in the wire.

The resistor values are arbitrary, but a too small wire may cause the assertion of having a negligible resistance to be false.  You can see on AWG page on Wikipédia the resistance of a cooper wire per meter as a result of it gage (diameter).  You have to consider that the higher the current, the higher the resistance.

In order to solve your problem it is you case, you have to:
1) Use larger wire (smaller gage);
2) Use shorter wire (if possible);
3) Avoid daisy chaining your boards, as the first wire will have a larger amount of current flowing that the last one, resulting in larger losses.

You should prefer a star configuration:







simulate this circuit

so that you have a different wire starting from your source to each of your board instead of a wire running from power supply to first board, then from first board to second one and so on.  Same thing applies to your return path (ground).

You may or may not be able to apply all of the previous advices depending on your setup, though.