I thought at first this was a bug in LR4; turns out it isn't but I thought I'd mention it anyway. One of the ways to link images to the map is to use a .gpx tracklog created using a handheld GPS or a GPS-enabled device like a phone. I have an android phone, and I have 'My Tracks' from Google installed on it. Went out today, walked five km and made sixty images. Exported the .gpx file from My Tracks, added it to LR, and synched the images. No matches. After puzzling it out for a bit I noticed at the bottom of the map that it showed the track as January 18, 2012. Today is the 17th. I used the time offset in LR to back up the tracklog 24 hours and all 61 images synched. The phone knows what date/time it is, and the .gpx file is labeled with today's date but for some reason the tracklog itself was ahead 24 hours. I read through the .gpx file to be sure. Anyway, I sent a message to the My Tracks support people at Google, but if anyone else is using My Tracks on an Android phone you might want to be aware of this.