Digital BW, The Print

I'm not consistantly getting good sharp scans. I cant establish a 
pattern to the behaviour. The negs are pin sharp through a loupe but 
scan soft. I know I have to apply sharpening after the scan but this 
is different. Some negs scan sharp and then just need that bit extra 
sharpening, and others are really really soft although they're 
actually sharp. 

I use Silverfast Ai 6.2.2 and have the little focus icon (looks like 
an enlarger) on so it shows bold and the 'crosshair' appears over the 
prescan image. The firmware is up to date.

Could the focus mechanism inside be loose or the autofocus be out of 
calibration ? Has anyone else had and solved this kind of problem? 
Anyone know how to make the adjustment ? I'm pretty handy and could 
take it on with a little instruction. Love to get an email from you - 
I'm tearing my hair out.