It looks like there is a gap between the videos and that's why they cannot be stitched together. Place the timeline Indicator on the tracks to the right. Holding down control will grab all the media to the right, then just slide the videos to the left to rejoin them back together. If you need to expand the Timeline to increase the size of the gap and make selection easier, do that by using the slider shown. Then you can stitch them back together. See Image Below, Click to enlarge.