Do you have the American-spec plugs in your car, or the J-spec plugs? American-spec spark plugs are just slightly shorter than the J-spec, so they just barely fit on and snap...this could be part of your problem. As for another possibility -- Engine vibration could be worse at the back of the motor than the front...? Defective wire that won't lock? Could be any one or combintation of those I'd say.