There are four screws to attach the plate to the housing, so the issue you described would just be due to not yet installing the remaining screws. The lower two attach to the bottom two tabs of the R2 plate; just make sure to use the included washers with those two (and only those two) screws. That is pretty much the only difference compared to the 87U install.
I hope this helps, but just let me know if I can answer any other questions or be of any other assistance.