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Make sure you only have one program trying to access the serial port at a time, i.e. if you are running UE4 close the Arduino serial window and vice versa. I would suggest using a “branch” and print to screen whether the serial port is in fact open in your blueprint. “Is Serial Port Open” will return true if the serial port successfully opened and false if it did not. Also do normal checks to make sure you are telling UE4 to open the correct port, I have commented on this before near the bottom of page 5.
With your Arduino Sketch, you should only need the serial.println() function because the plugin looks for a new line to read in data.