It might be because the group message is too long. From my experiences, group message text boxes allow up to 201 characters, but will only send a mail of 200 characters or less. Should you try to send a mail of 201 characters, instead of giving you an error message that your letter is too long, it'll simply close the window without any warning that the mail was not sent (and may still possibly cost 2 speakers, unfortunately. But I'm not sure on that).