Not all IPTC fields are indexed and searchable in catalogs. The individual photo will still contain all the metadata you've added to it, but you won't be able to search for that metadata in the catalogs. To see the current list of searchable fields, go to Catalog> Searchable fields...

We may add searchable fields to our index in the future. However, if you would like to search for a field that is not currently indexed, there is a simple workaround. Use the power of variables to add the missing fields to your keywords

For example, ISO is not currently indexed. Append ISO={ISO} to the keywords through the metadata (IPTC) template

You can now filter, browse, and search for specific ISO values within the keywords list.