OpenPGP error

[Sean Wilson]

Why is it when I sign an email and someone replies to it I sometimes get
the following error:

Part of the message signed; click on 'Details' button for more information

in the details it says:

OpenPGP Security Info

Error - signature verification failed

gpg command line and output:
C:\Program Files\GNU\GnuPG\gpg.exe
gpg: Signature made 10/11/09 14:13:48 using RSA key ID xxxxxx
gpg: BAD signature from "Sean Wilson <sean at xxxxxx.com>"

Why does this happen?

If I send an email between two different email accounts and I sign it,
then reply I NEVER get a broken signature so why does this happen when
other people reply to my emails?

Thank you!