I looked again in WHM and you first look at this setting:
Click on "List Accounts"
In the far right when the page opens, you will see at the top of the accounts lists right next 6to the "Cpanel Theme" , "Reseller" Look there to see what reseller user the account is located under. If its under another one of your users who is an authorized reseller, then from my own experience this is why you don't without logging directly into the VPS or Dedicated Network Admin control panel or whatever you have (depends upon host network), then this explains from my perspective why the account does not show up.
Try switching the owner of the account over to your WHM user name. On the other hand you could have a problem if the account iteself is NOT showing up on your defult WHM account user name in the accounts list you could have a bit of a problem. But, the wrong answer is deleting the account and re-creating it.
However, if you do create the account again after deleting it (I still don't recommend), here is strong advise for you to login to your own cpanel and back it up. Remember, from your own cpanel of your master account you type in only http://yourdomain.com/cpanel
Making sure you do NOT add the www. after the http:// header =+ ` because you will want to use the drop down lists that appears in your cpanel at the top that says "Manage another account:" This will allow you to get in your customers accounts and you can download a "home directory Backup' from there which will backup everything. Then delete the account in your WHM or locate the authorized reseller its account may or may not be under by mistake and terminate it. Again, once you've created the account again, you simply login to the cpanel of the customer account or back in your own and goto manage another account, simply upload the backup .tarz.GZ file zip and wam! that should take care of things. Unless you really do have some kind of screwy issues with your WHM eh..