Lorenzo,
I assume you're using the Macro+ mode. During my experimentations with the Z990 and this Macro+ mode, I noticed that the extremely short field depth can pose a challenge with focus.
To mitigate most issues, I do use the spot focus mode to make sure focus happens on a predictable point on the subject. I do suspect that, sometimes, even with spot mode, the focus is not exactly where I thought it would be.
Also, I noticed that the slightest movement from the subject can actually cause focus issues, which is totally understandable.
Very nice shots. Thanks for sharing.