If the Summicron that came with the camera is original to it, check the glass for yellowing. Summicrons with a SN earlier than about 105xxxx used Thorium. It tends to yellow the glass, but can be sun-bleached out. If you do this, you might also need to relube it. Or just use with B&W and skip the yellow filter!
These Barnack Leica's really grow on you. Fun to use, great little cameras.