Disk defragmenter query?

I ask the Disk Defragmenter to analyse my C: drive. It says "You should
defragment this volume". So I defragment the C: drive. Then I ask the Disk
Defrgamenter to analyse the C: drive again. But again it says "You should
defragment this volume" ...

I've been round this loop a few times now but I cannot get the Disk
Defragmenter to report "You do not need to defragment this volume". I've
tried rebooting between attempts to defragment, but it makes no difference.

Any ideas please? Am I encountering some limitation of the tool?