I spend quite a bit of time with these things to make sure the plates are fully seated. Linkage may need adjusting to allow them to fully close, and you may need to turn the plates slightly to get them to align well and fully seat in the bores.
I use a screwdriver handle to gently tap the plates into the bores and hold it up to the light before tightening the screws. If the plates are well sealed up, then you see more light after tightening the screws, it may need linkage adjustments as they are pulling the plates open.....Cliff