Still having problem with Calibre to GMail to Kindle

Hi,
This is so frustrating. I cannot reliably "send" a book from Calibre to my Kindle. Once in awhile it works but by and large it does not.
1) It appears that Calibre is emailing the book to my GMail account correctly as I can check my inbox in GMail and see that an email has come from Calibre and has an attachment with it (I.E. The book I am trying to email).
2) I've authorized receiving books from my GMail account on the Kindle management page in Amazon.
3) But the book only rarely or never seems to "reach" the Kindle email address.

Actually this whole thing is rather confusing to me. If I am emailing a book to my GMail account how does the Gmail account "send" it automatically to the Kindle email address? Shouldn't I be just sending it directly to the Kindle email address? How does this whole thing actually work?
As you can see I am confused.
Also what is the difference between my GMail address and my GMail username?
Aren't they one and the same?
Thanks for any help,
-Dennis