Computer PSU 5v Output direct with Raspberry Pi USB
Your five volt power isn't really five volts.  The newer Pis will run but turn off the LED if the power in drops below 4.63 volts.

Check the 5Volts where it connects to the Pi.  You may have losses in the wires from the power supply, and the power supply itself may just put out a slightly low voltage on the 5Volt rail.  Given the way you have things wired (multiple plugs and jacks and a breadboard in the middle,) it isn't just MAY have losses, it is rather WILL have losses.  (Thanks to @pipe for pointing it out.)

Info from here.  There are links in that post that lead to more info.