I managed to fix the problem. It turned out I had the "key" in the database as an AutoNumber, but everytime I was looking for it in the program I was using a String.
My program now more or less works fine, except for one little thing. In the part of my program where I create a playlist I want a message saying "No such track" to appear in the main textfield (called "list) if the textbox where the user inputs the key (trackNumber) is empty, or has an unrecognised input; basically if it is 'null'.
if (trackNumber == null)
//If the number doesn't correspond to anything in the library...
list.setText("No such track number");
//Display "No such track number" in the "information" text field
Except when testing this instead of the message I get "null - null".
I found that the message did actually appear whenever I changed "trackNumber" to "name" in the If statement.
( String name = LibraryData.getName(key); - LibraryData is the part of the program interacting with the database )
However, this then stops other parts of the program working - such as the reset function, which merely clears "list". The message "No such track number" just stays there.