The most likely reason that you can't find an athlete when you search in Game Desk is because they haven't created an Athlete Profile yet. There is a difference between simply having an Account (user) profile and having an Athlete Profile. An Athlete Profile needs to be created separately after you register for an account. 

Read this article to learn the difference between Account and Athlete Profiles

So if you can't find an athlete, reach out to them to make sure they have an Athlete Profile. You can also request that they share their 6-8 ID number with you so you can search by that instead of name. An athlete can find his/her 6-8 ID number within their Athlete Profile under their picture and name.