This is likely happening because your league doesn't have this position added in the My Playbook league settings.
Note: You can not do this via mobile, you must use the web version of My Playbook
In order to change your league settings, do the following:
1. Open My Leagues in My Playbook (https://www.fantasypros.com/nfl/myplaybook/edit.php);
2. Click on the Settings button for the league you're having issues with;
4. Adjust the roster settings so that they match your league settings;
5. Save your settings
Next, refresh your leagues and the players should appear for the position in question.