until now i did not have any problems with focusing.
The only thing that changed is the amount of magnification and the time.

I did a test yesterday with ortho film so that i could develop it fast and saw indeed that it was sharper(better focused?) when looking at the airimage in stead of the groundglass. I am now wondering it the time (2-3 minutes) also could have an impact since the whole image is not sharp. When different focused, there should be somewhere a sharp point. But in stead the whole image looks unsharp. I always close the windows and stand still during exposure.