I am not familiar with your setup, but let's try a couple of things. First, O see you have Facebook installed. That tends to mess with the default mail settings so first, go to Options > Advanced Options > Default Services > Messaging > What is the default account set to? Is it the account you expect?
If that is okay, with your Messages Inbox open, press ALT + O. Do you see any of the missing messages?
If not, with the Messages Inbox open > BB Menu key > Options > What is Hide Sent Messages set to?
If none of this helps the problem may be something in the way your IT Department has the email system set up. I am assuming your are not connected to a BES System because messages would not be sent to a Sent folder as far as I know.
Are you connecting to an Exchange Server? Do you use Outlook as your desktop email client? There may be something wrong with folder redirection.
See if
this BlackBerry Knowledge Base article gives you any help.....