I agree that one does not look to have gone in far enough, but they should not have to be forced in.
There is a knack to it, rotate the hub so that the thicker part is out of the way and one end goes in and you slide it up and then the other end slots in.
You may have to sand some paint off if they have just been painted, or rust on an old one just to clean both the sides of the slot and the front and rear of the backplate just a couple of mm where the cylinder will slide. I use a small amount of copper grease as much to try and stop rust as to lubricate. Before the shoes are fitted it should move up and down in the slot quite easily. In use one shoe will tend to hit the drum before the other and the movement then allows it to slide and centre so both shoes hit the drum equally. One then works harder and wears more as one is a leading shoe and the other a trailing one.
Also if it does not slide properly you will have issues with the handbrake, which actually has to be better than a modern car for the MOT.
If they are pattern parts you may have to smooth off a casting flash but I have never had any trouble fitting Lockheed Girling ones.