The might be a general bug with the close-up views: I had a similar problem with the pictures of happy customers in the living room when playing Gromit in the third chapter: I suddenly could not select any of them any longer! - Fortunately I had saved a bookmark at the beginning of the third chapter and when I resumed the game from there, everything was ok. Others have reported that they are stuck in such close-up views and they cannot get back to the normal game play. I could imagine that these two problems are related.
Anyway, unfortunately I guess that that you cannot proceed with the game in progress. The magazines should be always selectable, no matter what else you have done or not done before. And this is not about "pixel hunting": the sensitive area is very large. - I recommend to start from the beginning, save the game at the beginning of the third chapter and then frequently until you have passed that riddle.