Variable power.

I am sorry, I see that my msg was unclear. I meant to say that the spark plug itself is not loose; rather one plug wire is not seated in the valve cover and I cannot reseat it. The end of the plug wire appears to be covered in oil as the last 6 inches of the plug wire go into the valve cover. It feels like there is a guide that picks up the spark plug wire as it is being inserted into the valve cover, but I cannot push it down hard enough to completely seat it, either over the top of the plug, or finalize the seat in the valve cover. That loose connection may be causing the cylinder not to fire. I mention also that the design is unfamiliar to me as I have not before seen a plug wire going through the oil saturated area protected by the valve cover and I wonder how the oil is prevented from affecting the current. I can see the end of the plug appears to have oil in it when I remove it so how can in be inserted into the valve covered oil area without getting oil on it and how is it to be seated when I cannot apply enough force to push it over the plug from 6 inches away from the end?