This is what PS does when it can't find a match for the image - it puts it in its own little area on the canvas. PS used to have a window in the photo merge interface where you could manually position your images prior to letting PS go to town merging them - no longer. You may want to try to perform the merge without the left most image and let PS do the merge automatically, preserving the layers and masks that it creates.
Next, duplicate the merged pano and flatten it. Working with the flattened duplicate, extend the canvas on the left, where the orphaned image should be merged. Place the orphaned image roughly where it should be located in the panorama. Choose both layers (the flattened, merged pano and the orphaned image) and select Edit > Auto-align layers. Then do a Edit > Auto-blend layers. You may get an error if the image does not overlap the adjacent image "enough". If this is the case, try this same approach, but do it with the original pano still in its layered and masked form, and try selecting the orphaned image layer and the layer adjacent to it as the two candidates for Auto-align and Auto-blend.
I see you are using CS5 - I seem to recall that the 32bit version of CS5 still had the manual image placement feature ("interactive layout") in the pano interface ... maybe? I could be totally wrong here, it has been a while since CS5.
I apologize if CS5 does not have the features I am suggesting here. My version of CS5.1 Extended does have them (auto-align, auto-blend). Anyway, maybe this will work for you without having to resort to PTGui.
One of the many features of PTGui is that you can create a merge from your small JPEG renderings, get the stitch perfect, with no errors, etc using small files for fully interactive editing and stitching, and then save that stitch as a template. You can then open the full-res images (in the same order as the small images) and apply the template - the stitch should be identical, but now at full-res without all of the control point calculations having to be performed on the full-res images. This saves time and increases the interactivity of the stitching exercise without bogging down your machine.
kirk
