If the crank was used when the bolt was loose that could cause some wear on the spline of the crank or axle and that would require a thinner spacer to make a tight joint. You can check to see if a thinner spacer is needed by doing this: The ISIS specification; there shall be a press fit connection between the male and the female tapered spline of about 3 to 6 mm, normally around 5 mm. In practice this means, when you attach your crank arms by hand, there shall be around 3 to 6 mm space between crank shoulder and spacer (if not: take a thinner spacer!). Then you tighten it till there is no gap between spacer and crank left to reach the proper amount of press fit and then up to about 40 Nm which will tense up the screw enough so that it can not loosen itself.
From here.