In a VBM analysis I am interested in the negative association between a clinical score and GM volume in four different roughly equal-size groups.

I created a full factorial model in which I can test for negative associations between the clinical score and GM in the individual groups using four separate t-contrasts and run an omnibus F-test over the four individual contrasts. In one of the four groups a t-contrast shows a strong relation between volume and the clinical score in a large brain cluster (whole-brain, P<.001, FWE-corrected). In the omnibus F-test, a similar cluster can be seen at P<.001, but this cluster doesn't even come close to surviving FWE-correction (P=.4).

I had a discussion with my PI about this, he says the effect from the t-contrast probably doesn't show because the variances are unequal between groups, which makes the F-test invalid. My feeling is that this is not an issue, as the contrasts I am adding to the F-test are not comparing groups. Can anyone comment on this and venture a guess as to why the cluster that shows as very significant in the t-test doesn't even come close to surviving FWE correction in the omnibus F-test?