Floppies can fail, or seem to fail, for many reasons. Some are:

1) The oxide layer (the brown stuff) has flaked off because of age.

2) The oxide layer has been abraded off (e.g., by a scratched or dirty head).

3) The drive that wrote the data is misaligned relative to the drive that is reading the data.

4) The drive trying to read the data has a dirty head or heads.

5) The mechanism that "loads" the heads onto the disk surface is balky, leading to poor contact. The grease that lubricates the mechanism often turns to glue over time. The fix is to remove the old junk, replace with new lube.

6) A head has been mauled, leading to an "azimuthal" misalignment that cannot be repaired in most cases.

7) Media-detect switches have failed or gotten stuck, misidentifying the media type to the OS.

8) The "disk is inserted" switch has failed or gotten stuck, failing to let the OS know that a disk has been inserted.

9) The spindle motor can fail (either completely, or simply run at the wrong speed).

10) The head assembly motor can fail.

11) The cable assembly can be flaky.

12) The power supply voltages can be out of spec.

And the list above is hardly exhaustive (but exhausting to read, perhaps).