Richard is spot on, the Agfa bellows are notorious for developing light leaks, except for some pre WWII models that have nice leather bellows and some of the later top notch models (part of the Isolette Mk III with Solinar and Super-Isolette) that have improved bellows, too. Anyway, in most cases it is not absolutely necessary to exchange the bellows (in fact, as most Isolettes with Agnar or Apotar lenses are worth close to nothing I would consider new bellows only on the models with a Solinar lens). Instead you can seal the corners of the bellows' folds (that's were they usually crack) with some type of liquid black silicone sealant. It's not as beautiful as a new bellows but more in line with the value of the camera. This method works quite well for me and is rather durable. If your pictures still lack contrast after you have the leaks fixed and the lens cleaned ... well... the Apotar and Agnar lenses do have kind of lowish contrast, I would recommend to use a push on type shade at all times. The Solinar is much more contrasty and also quite sharp (with the others you have to stop down and there is a lot of sample variation). The Solinar does very well on modern slide film, and frankly, it's the only lens I personally would bother with on an Agfa Isolette, but that's ultimately a question of individual taste/preferences, of course.