Before you changed your plug, did you clean the dirt from the area around the plug cap and the cavity that the plug sits in? If not, there is a good chance that dirt got into the cylinder when you removed the plug. Dirt and water can collect under the plug cap if the plug cap is not sealed properly or is damaged. You have to take compressed air and blow the dirt out before removing the plug. Also, make sure the little drain hole at the bottom of the plug cavity is clear. This lets the water out.
Do a leak-down test to check your compression.