When screwing your tank into the marker do not force the tank to go in as you will damage the tank and asa. It should screw in smoothly.
While there is still air in the tank there is increased pressure holding the tank onto the marker. You can decrease this pressure by fully draining the tank or dry firing the marker with no paintballs after unscrewing the tank slightly to use the remaining air up. To do this turn the tank out ⅛ of a turn and then shoot about 3 times and then repeat and until the tank becomes loose.