In Lightroom 5, in the maps module, if I load a GPX tracklog I rarely see "auto-tag photos" become clickable. This is with matching timelines and photos that should line up. If I open the same GPX files and the same images in LR 4, it works just fine. In fact this is my work around at the moment. I will do the work in LR4 with sidecar files being autosaved. Then I will re-sync metadata in my LR5 instance.