I'm going to take a wild guess.
Each pin has multiple LEDs on it that spell out each letter?
Remember that the Arduino pins are only rated for 40mA max. If you are trying to drive a number of LEDs in parallel then your poor Arduino just can't keep up. If you are driving them in series, then the voltage across the string would have to be much larger than 5V.
If, indeed, the LEDs are in parallel be sure that each LED has its own individual current-limiting resistor and place an NPN transistor from ground to the cathodes of the LEDs and then place the current-limiting resistors from the anode to +5V. Use a 10k resistor from the each Arduino pin to the base of each transistor.
With 330 ohm resistors, each LED will draw about 10mA. So a 2n2222 should be fine. It has a max current of 200mA so it would drive 20 LEDs.
I hope that this helps but it is a wild guess.
