Outside miters in flooring makes no sense. I take it, then, that maybe you mean baseboard?
18 ga. brads are more likely to have an effect than 21 or 23 ga. pins.
Casey

So the outside miters don't meet up because the floor is not level? I would recut the miters (maybe on a slight angle, too) to get them to meet more tightly. Little bit of trial and error. Glue and nail, then caulk the wall gaps.