Why Won't This Fillet?

 From:  OSTexo

I'm building an AC adapter using a combination of MoI and SCE. I am trying to fillet the back of this to 3mm. For some reason this doesn't work too well in SCE or MoI. Interestingly it works when I set to 3.01mm, but not 3 exactly. For some reason when I apply a 3mm fillet to the entire model it works fine, but I am unable to use the variable radius fillet in SCE. Thoughts? Thanks.

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