stly make sure that you’re using the focus ramp (component A4) properly! The focus ramp is, as the name suggests, a ramp that is thick on one end and thin on the other. Sliding the focus ramp from left to right changes the distance from the lens from the sample, shifting the focus. To start at the lowest focus, put the thin side of the ramp between the lens and the sample. To adjust focus, push toward the thick side while viewing your sample until you can see the sample clearly.
Secondly, make sure you are viewing objects that are translucent, or allow light through. Completely opaque objects can not be viewed using transmission light microscopy.
Third, errors in the assembly process can result in difficulty focusing. Please review the videos at https://www.youtube.com/playlist?list=PLnO8NcEb6LFy1p0rCVBfHY0JUgBswRC4m to ensure you have properly built your Foldscope.