I think, the problem is, that many of my components are indeed nested, as you write, so the way to solve the problem must be to make copies (instances) of these, that are not nested. Then these instances may be deleted, but the components will remain and then be globally available, right?

Sometimes only a few of the components in my models are available in the component browser. What is the reason, and what might I do to fix this?

Apparently some warehouse-models are made in such a way, that they easily cause Sketchup to crash. There ought to be warnings about this. But of course, It was silly of me not to remenber to SAVE BEFORE LOADING, as one always should. Just lost at least two hours of work