how can i fix my double glazed door lock?

On my front door, the door will only lock on the inside but not on the outside. so everytime we leave the house we lock it from the inside and use the back door. does the barrel need changing or is there a very simply reason this is happening. if the barrel needs changing how do i do this. Iv taken out the two screws from the inside panel of the door but i cant take out the barrel.