This can happen because your client or athlete hasn't given you correct permissions to his account. To send him documents or programs you will have to have "Create/Edit" access to his account.
If the client's account has expired or if he does not have a Pro account he will not be available to send documents or programs to.
Another situation that could cause clients to be missing from the Send window is if you are a trainer working inside another account and that account does not give you Create/Edit access to Workouts of the clients.
So if you're working inside another account, please ask the administrator of the account to check on your permissions and make sure you have Create/Edit for Workouts (individual training sessions).