If your dimension is a discrete variable like company or sales rep names, Tableau won't provide values as it can with "show missing dates." Dates and times, states, countires, etc. are special kinds of universal dimensions, which is why it is possible to autocomplete incomplete lists.
What I find easiest to do in these situations is to create a "master roster" of all possible client, sales rep and/or dates/month (technically a CROSS JOIN in SQL speak) combinations that you would ever want to see.
Are you able to post a packaged (.twbx) workbook? So Your Question Didn't Get Answered...