#127 Magmi doesn't remove the association with some categories

[mageLuke]

Hello,
I don't know if it could be a bug of Magmi but I'm facing a problem with product categories.
In few words, when I try to import a product with a new category association (I use "categories_ids" column) Magmi should remove the previous associations and add only the ones specified in the coulmn.
Although the categories have the same values for their attributes (active, anchor...) it seems this doesn't work for some of them and I don't know why.

Have you ever experienced this kind of problem?
Where should I see to find the reason of this problem?

Magmi version: MAGMI 0.7.18
Magento version: 1.7.0.2

Thanks for the support.