Too much glare. If you can see a thin split on the bottom serif it might be. 1968-S cents are notorious for strike doubling. If there is any doubling on the date it is probably not an RPM. Try to take a photo using florescent light to cut the glare. This probably will not work with your camera "rig."
What you need to look for is a very small split or "notch" at the bottom tail of the mint mark. These were notorious for partial or full filling of the bottom of the MM, just poor strikes in general. I also see what you are but this looks more like MD than anything else. I have found full rolls of these all with the lower portion filled but they are not an error. They are also very hard to photograph w/o the proper equipment or set up.
Here is a 1949D Lincoln (photo can be enlarged) , not the best photo but the only one I could find in a rush.