You can modify any "use values from field" based value list whether it is conditional or not by editing the values in the table from which those values are sourced. You might add a button to your layout or an "action value" (My term) to your value list that opens a window displaying the records for editing. Or you might open a dialog box with an input field or fields where the user enters the values for the new value record and then the script uses those values to create the new record. (an "action value" is a special value added to the list that is detected by a script trigger performed script and when detected, causes the script to perform a special task instead of entering a value.)
The most likely reason that your value does not appear is that you have a duplicate value in the other field that makes up a two field, field based value list.
example: You set up a value list where Field 1 is an ID number and field 2 is a customer name. You have two customers named "John Smith". because you sort your values on the name field, only 1 "John Smith" appears in the value list and you cannot select the second John Smith in order to enter their ID into the field formatted with this value list.
Another possibility is that your relationship is not matching values the way that you expected them to. Since it appears that you are using text fields to match values in the relationship, the inclusion or omission of a single space character can keep the values from matching when appearances would mislead you into thinking that they matched.
I have just created a free to download instructional file called "Adventures In FileMaking #1" it demonstrates 8 different variations of conditional value lists with detailed information on how each was set up and how they work. You can download that file if interested, from my FaceBook page (See link to follow.)
PS. "action values" are one of the topics covered in Adventures In FileMaking #2, a file that I currently have undergoing development...