2002 Nissan Altima/Spark plugs getting wet

I have a 2002 Nissan Altima/2.5. I keep having problems with one of my Spark plugs getting wet by oil. I have changed the same plug twice. But it continue to keep getting wet. I called the dealer and other parts stores. But no one can tell me what may be causing the oil to get to the spark plug. maybe you could help me.

Thanks in Advance.

-LaRon Kelly :D