We've had ia mixture of different relationship styles.
Breathes has had one relationship where I could not, still can't, stand her. It's very difficult to keep the lines of communication open where you really don't want to deal with that person at all.
He's had a relationship where I was just starting to get to know her but she decided to date someone else.
I've got a relationship where all of us (his partners & us) will get together for a games night or a picnic or something & we all get alone really well together.
We've had the beginnings of a triad but things didn't work out although we all got along well together.
It all depends on the persons involved. If they have things you can like or admire, things in common, then there is a really good chance you can have some sort of relationship with them even if it's just being able to discuss different things about your mutual partner together.
Personally I think PN & Mono are doing exceptionally well. From what Mono has said they will go out for coffee and discuss many different things.
Sometimes we, as the mutual partner, have to just step back & let things develop as they will. It's difficult to do because we want them to get along so very badly. It is so worth the effort though because they get the chance to learn about each other at their own speed, they can communicate without a mediator and they can support each other in their times of need.
I don't think that you're a bad metamour so much as you just haven't found one of PN's other partners compatible with your needs. There is nothing wrong with this. Just leave the lines of communication open, even if it's just via email, & it will all work out the it should.