GND polygon in Altium
My question is why some GND Pin aren't connected to the Polygon?


Because your clearance rule does not allow the polygon to reach the pins. Try reducing the clearance and see how that changes.

Also, you have unconnected islands in your polygon (called "Dead Copper"). This can cause you issues, as the polygon looks good, but is not actually connected to anything. You can remove these islands from the polygon properties menu.