Why is Keynote changing my black text to white in the PDF?

Bit of an odd one - when I export a Keynote document to PDF, it's turning some of my text white. There seems to be no hard and fast rule about which bits of text change colour (i.e. different fonts), but it is always the same bits of text.

I've tried:

• 'Printing' to PDF instead
• Copying the style from text which comes out ok onto it
• Retyping from scratch

... all to no avail. Retyping changes which bits of text come out white.

The only workaround I've found it to export to Powerpoint and then export to PDF from that - obviously not ideal.

Any ideas?

Thanks