Line Graph Data Point Markers Disappearing

Having an issue where when I update the data in my line graphs, the data point markers (green squares) disappear and I cannot get them to come back. Even after re-applying the custom data point marker through the graph design menu. 

How can I ensure that the design of the graph stays consistant after I update the data?

Before Updating Data:

After Updating Data (Data Point Markers are Lost):