I remember this puzzle being kind of picky.
There are hidden drawings and letters on the fresco. Move the lit candle on the fresco very slowly. When the start of the hidden drawings starts to be seen, move the candle very slowly to highlight all the parts of the hidden drawing. At a certain spot in that drawing, the sketchbook will appear at bottom right of the page showing that Gus drew that particular hidden drawing and should not appear anymore. If the sketchbook does not appear move the candle over again very, very slowly and sometimes hold it in place to get Gus to draw it. Patience! The yellow X in the picture below is approximately where the sketchbook shows up (refer to the picture in the walkthrough for this - it may help you).
When all the hidden drawings and letters have been sketched by Gus, the sketchbook will automatically appear on the screen.
Patience, and being really slow and thorough, is the best advice I can offer.