If you have not got enough thread sticking out from the struts to do up the nut properly then it sounds to me like the thread hasn't been done up enough before the strut was fitted back to the car.
As suggested above remove the top cap and nut and washer and you should have a rubber top piece, remove this and theres another washer and a nut underneath that holds everything together.
Try doing this up as it may have come loose.
Also when you remove the top rubber is the lower one, in the turret from underneath, correctly lipped over the little metal flange in the turret. I've had this before where it wasn't sitting in properly.
