As mentioned above there are two types of floppies out there.

Be aware that there is an extra hole in the casing of the high density version.

A small number of floppy disc mechanisms detect this hole and then assume that the floppy disc is firstly of the type thus indicated and secondly IS FORMATTED TO MATCH. As a result, if you put a cross-formatted disc (either a 1.44 meg disc as a 720k, or a 720k one formatted as a 1.44) and it will not be able to read it.

It may be that your guitar centre discs are DD formatted but in a HD case. If theres a hole "top right" of the disc (looking at the label) where the "read only" hole (with the sliding closure mechanism behind it) is top left, then put a piece of tape over the "extra" hole and see if that helps!

The discs may be DD formatted because the first hardware sequencers / midi file playback devices used DD mechanisms.

Side issue: cross formatting is not recommended; despite what you may read on the internet DD discs are not just HD discs that failed the quality check, the specification of the magnetic material is different as well.