John,
I have always avoided targeting directories of files when creating IDM's because it's very rare to have enough of those files structured to a point where an IDM would make sense. I always recommend trying to find some commonalities between the files for example, find a directory with a certain amount of M&A documents but ONLY M&A. Combining different types of documents would only decrease your accuracy score.
Also, you may come across a spreadsheet that contains images and multiple sheets and/or password protection/archived which your IDM would then convert over to an exact match IDM which would mean that you can only generate incidents on an exact match. This also takes place if a file type exists in the directory that Symantec cannot detect. If the file type cannot be determined by DLP, then it just looks for the binary makup of the file types.
My recommendation would be to comb through the directory and make sure that you can find common file types with no encryption or archival and avoid unrecognizable file types like ISO's and JPEGS.