And if I remember correctly, the motherhouse for that order was somewhere in the Midwest.
Or there could have been the situation where the land was sold, the graves remain with markers, but as time went on the markers were removed or destroyed. I am finding that some states, because of low populations 100 years ago, had somewhat lax standards, because no one could see future development eating up the land in the rural area.
Another idea, if you have any idea of who your relative was with in the convent, is to try to contact those families. I have had to do something like that, and I got very good clues and results.