This may be a stupid answer and maybe at the enlargement percentages most of you are doing it doesn't make a difference, but with the extreme enlargements I do, I found that my focus would appear to be dead on, but the print would not be sharp. Well, I finally figured out that I was focussing on the print platform with complete disregard for the depth of the paper itself, causing me to be minimum of 2-4 mm out of focus. With film (my film are 7mm thick) it is much worse. Anyway if you think this might help, just apply a scrap piece of similar medium under you focussing tool when focussing, and see if it makes a difference.