OPF-FN509T, RMR plate has a tight fit on the pistol's optics pocket by design. Given the tolerances of the sight pocket, the plate can be tight. Below is how to install OPF-509T, RMR if your plate happens to be on the tight side.
Remove the slide's optics cover plate.
Gently (do not force it) place the OPF-FN509T, RMR plate on the slide, make sure the plate is parallel to the bore, the edge of the plate should be about midway on the slide.
Front edge in first, then tilt the rear of the plate down.
Make sure the plate's bottom can clear the raised section in the pocket
From the side, a partially installed plate will look like this:
Slide the plate fully on the slide, at this point, the plate should drop in place.
Below is a video on how to install a tight OPF-FN509T, RMR plate:
