To clarify, is this isis bb or square taper? If its square its possible the crank arm is worn, thus needing a new crank/cranks. If its isis, there should be a spacer between the "butt" at the end of the splines and the crank when its tight, as thats the way there designed to be, if theres no washer obviously you will be left with a gap, so yea, as I think was previously mentioned try and find a washer to go over the bb before the crank goes on. If that fails it would be worth trying to find a washer with an internal diameter larger than the bottom bracket axel, and put it between the crank and the crank bolt (obviously it needs an external diamter smaller than the counter sunk hole in the crank) this should allow the bolt to push the crank further onto the axel.