When exporting a file the message indicates Lightroom cannot find file or cannot open it (although I have checked the file in Develop Module). This is an intermittent problem - sometimes works and sometimesfails. I have not had this problem prior to upgrading to the latest Lightroom version (2015). I have been using LR since its inception. My platform is Windows 8.1. Please advise how to resolve.
It's a known bug acknowledged by Adobe. See this topic in the official Adobe feedback forum: Lightroom 6 does not export all selected images. Please add your vote and opinion to that thread to make it more likely Adobe will prioritize a fix.
Also, see this reply for a workaround: Lightroom 6 does not export all selected images.