Grouping by taxonomy doesn't work

Hello,

I'm using a view which is placed into a block using quicktabs module which displays some pictures that are to be grouped by certain taxonomy values.
I realized that after several pictures are uploaded, the view suddenly doesn't group them propperly (some pictures with taxonomy A are placed into a group of pictures with taxonomy B, while others are placed in Taxonomy A -where they should be). I changed several display modes and it's always the same error.

Hope there's a solution to this exellent work.

regards