Final Cut Pro X: adding transitions - problems with synching

After adding this blur transition in between the shot of Sophie's eyes and the trees we found that the lip synching of Julian became out of sync with the vocals on the track.

Originally the markers that we had added onto the original track and the film were perfectly matched on the time line. However, adding the transition appeared to move the video of Julian singing further on in the time line which then meant that his lip synching was not in time with the original track. This is extremely frustrating as it takes an extremely long time to perfectly sync these up on the time line.