Text on page disappears when applying slide master

I have an image on a slide master and when I apply that slide master to a page in the document and try and add text to the page, the text doesn't appear. The text box is there and it gives the impression of overset text. If I move the text box to a page with no master applied, the text appears correctly.

The screengrab shows at the top, the page with the master applied and you can see the text box with it's overset marker. The bottom, white page shows a copy of the text box as it should appear.

I can add images to the page with the master applied and have no issues.

Any help would be grately received as it's driving me mad!