Piston not updating

Is there any obvious reason why Piston would not apply changes when
updating plugins? It seems to go through all the motions when I run
"piston update vendor/plugins/my_plugin" but never seems to make any
changes - the last line always readys "Updated to r123 (0 changes)"
regardless of any changes in the repository.

I've tried re-importing into Piston, but it doesn't seem to make a
difference. Any suggestions?

I'm using Piston 1.3.3 and Ruby 1.8.5 on WinXP