[plt-scheme] Uncorrect enter! behaviour

I tried to follow the example in the document titled

'More: Systems Programming with PLT Scheme'

So I writed the serve.ss example as follows :

#lang scheme
  (define (go)
    'yep-it-works)

And loaded it with 

   (enter! "serve.ss")

After that, I tried to modify the example, but reevaluating the enter!
statement,
it doesn't reload my changes.

In order to see them I have to quit MzScheme and restart it.

I'm using MzScheme vers. 4.0 [3m], under Windows 2000 sp4.

Any suggestion ?

Regards
Maurizio.