When you put text on the bottom layers it has to be mirrored. There's two ways I have done it:
- When you create the text, type in what you want, then select the bottom copper or silkscreen layer before placing the text and it will flip the text automatically. or
- If the text already exists, change its layer to one of the bottom layers. If it doesn't flip automatically, use the 'mirror component' tool on it after you have changed the layer.