Why is Na5 not played in this line of the French Defense, Advance Variation? I find Na5 a good resource for black in this line of the french, profiting from the fact that white's b-pawn is binned to the bishop. As far as I can understand this position with my limited chess, I find it a strong move, winning space on queenside with the plan of trying Bd7-Rc8-Nc4 on c-file. 

[FEN "r1b1kbnr/pp3ppp/1qn1p3/2ppP3/3P4/2P2N2/PP3PPP/RNBQKB1R w KQkq - 3 6"]

6. a3 Nh6 7. b4 cxd4 8. cxd4 Nf5
9. Bb2 Na5


However, I don't find this move at the database of chesstempo I use to use for my correspondence games.



Stockfish neither considers it. It even don't play 10....Nbd2 (it plays Nc3) to defend on c4 (which I would expect), and doesn't wonder at all about Nc4 (it simply trades his bishop for the knight; apparently it should result an advantage on the center).

What's my misunderstanding here?
The Nbd2 --> Nc4 plan to protect the bishop doesn't work because c4 is attacked by black's pawn. Stockfish takes a more direct route to kick the pin:

[fen "r1b1kb1r/pp3ppp/1q2p3/n2pPn2/1P1P4/P4N2/1B3PPP/RN1QKB1R w KQkq - 0 1"]

1. Nc3 {White can kick the pin in a moment.} (1. Nbd2 {This move also poorly develops the knight - and as you can see, c4 is covered anyway.}) 1...Bd7 2. Na4


And the pin is already gone. Na5 didn't really do anything in the first place. It was interesting while it lasted, but that wasn't long. It wasn't exactly a bad move, even (lichess would call it an "inaccuracy," I'd wager); however, there are better ideas here, such as the simple developing move Bd7.