As marcoferrari has stated … the description field on the item ledger entry table is only updated if the description (through the posting) is different to the item description. This makes sense if you think about it since having the description shown all the time (even if it had not changed) would not serve any purpose. You would really only want to know when a user had entered a different description.
In my experience I have found that once users understand why the description field is blank (most of the time) … the original issue disappears.