As a general rule, your catalog should only contain one copy of a file. That file can appear in numerous albums or can be tagged with multiple tags. But if you try to import the same file, even from a different folder location, the file will be rejected because it is already in the catalog. To determine whether this is in fact the case, you can do a search for a file name (using Ctrl+Shift+K).
Now, if the file is missing (and you only have a thumbnail with a question mark available) you can delete it from the catalog. You should then be able to import the file from its new folder location.
If this is not your situation, please give us a little more detail.