Strange behaviour of NetEncryptString and NetDecryptString

[joep]

I use NetEncryptString and NetDecryptString to decrypt passwords. So they won't show there initial value in the database.
Example:
Usr:Wachtwoord = NetEncryptString(lWachtwoord,len(lWachtwoord),'xxxxxxx')
And to show it again or to validate the paswords
lWachtwoordString = usr:Wachtwoord
lWachtwoord = NetDecryptString(lWachtwoordString,len(lWachtwoordString),'xxxxxxx')

But not all of the paswords can be decrypted.
These are some values and the indication wheter they can be decrypted:
- AA34&&&^    yes
- AA$A8nt56Y   yes
- $$34er$         no
- A3gh2$          yes
- B3gh2$          no

Can someone tell me why some values cannot be decrypted?

Regards
Joep