I had this same problem. The enclosed screws for attaching the mounting bracket protrude past the rim/edge of the box (circled in red). When you place the plastic mounting plate on the box, it is impossible for it to sit flush. You can tighten to that center screw all day long but there will still be a 1/8” gap between the plastic mounting plate and your box. Please see attached photo taken from below - the screw is all the way in and I circled in red where it sticks up. 

I tried multiple things. I called Ring, but after spending time on this site looking for a solution and calling and being on hold for a half hour, my phone battery was almost dead and I had to get the project done. Here are the things I tried first that DIDN’T work:

1) Went to the store to look for screws with flatter heads that would be flush. Bought 2 different kinds - including some that were tapered to counter-sink. They still stuck out too far. 

2) Shopped for a different box that might allow the mounting plate to sit deeper inside. No luck. 

3) Tried to locate an additional seal ring to close the gap. Couldn’t find one. 

4) Tried cutting away the foam seal where the screws were. Still had a 1/8” gap. 

THE SOLUTION - I finally drilled holes all the way through the plastic mounting plate where the screw heads left an impression in the foam with a 3/8” drill bit. FINALLY, the mounting plate would sit snug on the mounting box (which was newly installed by an electrician) without having to apply a ton of caulk. 

I can’t believe after all these comments that Ring HAS NOT FIXED THE PROBLEM OR OFFERED A VIABLE SOLUTION. This should have been a 20 minute job, but with traveling to the store and up/down the ladder so many times, it was over 4 hours. I hope this helps someone else avoid the same frustration.