Having trouble reading excel file with the OpenXML sdk

I have a function that reads from an excel file and stores the results in a DataSet. I have another function that writes to an excel file. When I try to read from a regular human-generated excel file, the excel reading function returns a blank DataSet, but when I read from the excel file generated by the writing function, it works perfectly fine. The function then will not work on a regular generated excel file, even when I just copy and paste the contents of the function generated excel file. I finally tracked it down to this, but I have no idea where to go from here. Is there something wrong with my code?