One thing I learned about cartomizers is to make sure the little hole in the center of the filling is lined up with the hole at the bottom of the atomizer end. Hold it up and make sure you can see daylight through the whole way, and if you can't, take a toothpick and gently push the wadding until it is back like it is supposed to be. If it isn't lined up right, it won't draw, or the draw will be very difficult.
And when filling, tilt the cartomizer to the side and drop down the edge of the metal in one spot, then for the last drop or two, turn it around and drop on the opposite side, where there will be a dry-looking place. This will keep the juice from running through the cartomizer and out the atomizer end.
I do not see that there is any appreciable difference with or without the little inside washer doolie. Seriously--if there is a difference, it isn't enough to make it worth fooling with the washer.
I put the flat-tip shipping mouthpiece from an old 510 atty in the end, too. I like flat-tips, and the flat-tipped cartridges I have are too long to fit nicely. Not sure what kind of shipping piece is on a 902, but if it is one you like, switch that out for the little rubber plug and you will be a lot happier.