appearance of icons

When we are in pictures- the icons have the JPEG icon on them or the PDF, or PAGES. Those used to show the actual picture that could be seen if we clicked on that "icon." (maybe the correct name for these little boxes isn't icon???) The same sort of problem sometimes happens when we are in DOCUMENTS. instead of the image of what is actually in that "box" there is just the JPG icon, or logo. Is there a way we can change this? It hasn't always been like this. In documents, this doesn't happen in every folder. We have mac OS Monterey, vs 12.1 I don't know if I've selected the correct answers to the questions that follow this section.