fonts in dynamic images

Hi all, I have been creating some dynamic images, with text. Some fonts seem to work fine, but some give the right number of characters, but each character is just a square box. What's causing this. I know the fonts are there (there is an entirely different error message when the fonts are not there). I would just give up on them , but theres one font I like, that definately worked 6 months ag, but is 'doing the box thing' now Anyone have any ideas. Thanks in advance Jules