Digital BW, The Print

I have an Epson Perfection 2450 scanner operating on Windows XP.
The last couple of scans have not been sharp, despite my usual 
previously satisfactory techniques, that include scanning via Vuescan 
(with no sharpening)in 16 bit mode, to a print resolution of 300 and 
A3 size.One negative was a 5x4 Polaroid 55, and the second a 645 size 
Velvia 100 colour positive film.
I am using the standard supplied negative carriers.
Can the scanner be 're-focussed', or does the problem indicate a 
hardware failure?
Any ideas?
Many thanks
John Fontana