Drop Down Lists - How to make list items automatically appear when typing the first letter of the word

I am creating a PDF form which contains a number of drop down lists. In one of the fields the list item will automatically fill in on the form when the first letter is typed. For example, if the list contains HOME, APARTMENT, INSTITUTION and I type the letter"H" the list item HOME is automatically inserted into the field.
 
The other drop down list fields do not do this and I can't figure out what setting has changed between the two fields. I'd like this option on all the drop down list fields. Any ideas?