They are functional, they are properly referenced in the .dpr, but I needed to change a component for another type and I am not able to access the DESIGN of that unit. I access the DESIGN of some and not others.
Remembering that: I don't have any errors in the terminal, when I run the project, even the screens with the buggy design work perfectly. Only in development I can't open the DESIGN part to change components.
With this limited information, I have no idea about what might be causing this.
Are these just code units or form units with DFM & HTML file? Are the DFM and HTML file shown in the project manager? Have you tried to create a new project and add these units (and possibly DFM/HTML file) to this new project and see what happens?
Finally, what Delphi version is this?