Tables with Relationships Don't Filter as Expected

[sgross]

I'm testing out the relationship between two sheets, using a small sample size and dummy data, and I'm coming across some inconsistent behaviour that I'd like to fix.

One sheet, TITLES, contains the information for books (subject, publisher, etc.) and the second sheet, USAGE, contains a count of how many students have read the book each month. Both sheets are linked, in the Relationship Editor, by the books' ISBN.

I have a line graph, created using the USAGE table, showing the number of students that have read the book, and using the book's name as the legend. I've then created a slicer using the SUBJECT field from the TITLE table:

Now, if I select English on the slicer, the two English titles and their values remain on the line graph, as expected. But if I select any other Subject, nothing appears. The only way to see the other three books' information is to select (Blank) on the slicer, despite the fact that they are all drawing their Subject data from the same table:

Any ideas as to why this is occurring?

[KHorseman]

Hard to tell without seeing a sample of the data.

[sgross · Accepted solution]

In the process of grabbing some screenshots, I discovered the error.

Though the sheets recognised each other, one sheet's ISBN column was text, the other sheet's ISBN column was a general format, so the values were being truncated!