Gap between my solid objects when colliding?



Hey guys, I'm new to Gamemaker Studio 2.

I'm using the drag and drop features to create my game.
What I want is for the blue box to bounce back and forth between the two grey walls on either side.
I have the movement down, but for some reason the blue and grey box arent actually touching.
I want it to be a pixel perfect collision, but I cant seem to figure out why there is a gap in between them. This gap only appears on when the blue box touches the right wall, doesnt happen on the left.

I've tried changing the collision masks, turning both objects off and on solid, but I cant figure it out.

Any help would be greatly appreciated.