Improper fitment of the rubber boot will do it everytime, but a good test is to take a spare plug and try putting it in the cap, then pulling it out. If it holds then you're not seating it well enough, if it doesn't hold then you need to resolve it. Two options; either take the cap apart (just screws out) and rebend the little clips OR replace it. You can replace with OEM or aftermarket, I currently use an NGK cap and it does accept the rubber boot. If you do replace it remember to get the proper resistance, can't recall exactly what it is off the top of my head (5k maybe?) but most places only sell 2 kinds, resistor type and non resistor type.
