When you are shimming them from behind, you need to raise the pins with a pick to find the shear line between the driver pins and the key pins.
ASSA locks can be tricky with this because of the special pins they use. But it can be done. Sometimes I find it easier to lift the back pin stack all the way up, then keep trying to get the shim to slide in as I lower the pins stack.
If you are used to mounting the locks with the pins at the bottom of the keyway, then switch everything I said from 'raise' to 'lower'.
It may help us to see the keyway of the lock, as some ASSA locks are made differently.
Good luck!
Gordon
Just when you finally think you have learned it all, that is when you learn that you don't know anything yet.