Blue Tit Nest

 Earlier this year, a pair of Blue Tits took up residence in our nest box. They spent a couple of weeks going in and out with beaks full of nest material then they just seem to have abandoned it. I wonder why they did that? I have just taken the box down to clean it and found a fully completed nest inside with no evidence that they ever used it. The box was full of Earwigs, could that have put them off ? If so, how to keep Earwigs out of nest boxes?