Bill Springfield in Colorado Springs works on these.
http://www.triggerwork.net/fnh-fnxfnp,-fn57,-fns,-509.html
He's worked on many triggers for me - Beretta, Barrett, Sig, HK, Steyr - and his work has been good, plus he's accessable and ships your piece back promptly.
You also might try posting your question over on the FN Forum.
https://fnforum.net