Track Matte not working

After Effects, I make a Composition of 3 layers ;

- a blue Rectangle shape, which is supposed to serve as background for everything to be added over it,

- another Rectangle shape, no fill, placed under the blue rectangle,

- a logo (imported .PNG), which I set to start from within the blue rectangle, move downwards and only appear when placed within the second rectangle. I then set the logo to be track matted by the second rectangle, intending the logo to be visible while within the matte's area.

So, the logo starts from within the blue rectangle, moves downwards to be placed within the matte. Stays there for 5 seconds then back up.

No other effects are used. Elementary animation, nothing complicated.

And, indeed, it runs fine when I run it with the 2 layers (matte & logo). But when all 3 layers are on, the logo is never visible. Any suggestions ?