A/D nominal port interaction

Have you tried swapping the dim LED with another?
If the LED which you have moved to another place it still dim then it is a faulty LED.
If not then check the resistor value is actually the same as all the rest, if it is then look at the pin mode setup for that LED.

Otherwise post your code.

@wallup Installation and Troubleshooting is for Problems with the Arduino itself NOT your project. It says so in the description of the section. Therefore I have moved your post here.

You may want to read this before you proceed:-
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