Apple Mail - email contents disappearing on macbook air

I use apple mail and have several email addresses that feed into it. with the email address i use for work, I have created a number of mailbox files in apple mail. When I move an email addressed to me at this address into a mailbox, and I go to refer to it later, whilst the email is there, it's contents have disappeared and only the address, sime sent and subject appear. Can anyone help me?