There are values listed in a taxonomy, which you might have newly added, and now you want to select them from a dropdown list, but they are not there.
Check which values are shown
Open the information where you expected the dropdown to show the mentioned values. In the ‘General’ tab, in the ‘Options’ section, check if the option ‘Show all values’ is set to ‘No’. If so, you have to specify which values of the taxonomy should be displayed.