I might have thought the 1-sided vs 2-sided (or, for clustering, like "bi-sided") might be the issue, but it looks like the missing cluster had the same size as several others (6 voxels). Juuuust checking that there wasn't one cluster that would be seen by scrolling down in the cluster report? Also, knowing the threshold in each case would be useful (did you provide a p-value and use the internal p-to-stat calculation for each dataset?).

I notice the the peak-voxel locations are slightly different in some cases, which is slightly odd if these values should be exactly the same.

Do you have the clusterize settings you were using for each case? Note that in the cluster report of which you showed images, you can click the "3dclust" button above the table and output the 3dClusterize command being used internally---could you copy and paste those?

thanks,
pt