Limited directory tree width
When you press
File → Sketchbook you get the list of your project directories where you can select from.
If the number of entries in that list is too large the last ones cannot be displayed and not selected.
For instance when your monitor has a vertical resolution of 1024 pixels you won't see more than 46 projects listed.
So what you should do is find similar projects, create a new directory, find an adequate name for it, and move those projects into this new directory. When you now press
File → Sketchbook those new directory will displayed with a triangle to indicate there are more projects inside.