On the right, the spacer (lined in red) sticks out a fair bit, but does seem to be in as far as it will go:

The yellow lines mark the disc mount, which I've swung round in the pic to get it out of the way.
On the left, that doesn't leave enough space for the spacer (available space in red, section which needs to fit in it is marked in yellow):

In the pic, the spacer is rotated by 180 degress - flange should go the left and rest against the swingarm - I can't describe why I've taken the picture like that

Neither side seems to have any places where one of the bits should slot in, but I suspect maybe the spacer on the right should stick out a little less than it is doing.
Can anyone see what I'm doing wrong?