Marco, in the following post (#40), "Michael R 1974" explains things.
Originally Posted by Marco Buonocore
Let me try to be a bit more blunt. It is possible that your paper can be exposed by UV light, which you cannot see. The (invisible) UV image is probably out-of-focus. I suggest exposing through a UV filter, which will block the UV light, eliminating this possibility. This simple (if you own a UV filter) test can eliminate any need to hypothesize about whether or not UV light might be causing a fuzzy image.
I mention this possibility because the photographer/printer/writer Ctein reports that it has happened to him, and I do not take Ctein lightly. I don't have evidence that it IS your problem, only that it is a possibility that can be easily tested.