Sometimes when you change strings, the ball ends don't seat completely on the bridge plate. I usually gently pull up on them as I install them to insure they get seated. I'd start there.
You might also have old/oxidized/defective strings. Not common, but not impossible.
As far as bone, it looks different, but if you have the saddle out drop it on a hard surface. It will rang where plastic will sort of clunk. It also smells horrible if you sand it.
Martin to my knowledge never used "plastic". They did use Micarta for several years, post-ivory. Bone has been their go to for many years now.
I have bone on every single one of my guitars. It's cheap, sounds good, and lasts.
: Two actually. First: What is a double bound
: guitar body mean? I know what binding is but
: what is meant by double bound? Second, how
: do I know if I have a bone or plastic nut or
: saddle on any of my guitars, especially my
: Martins? And which is better? Might have
: nothing to do with the question, but I
: recently changed the strings on my D-18 and
: now it sounds dead. Not very bright and not
: much sustain. I put medium gauge Martin
: Lifespan strings on it, I believe I used
: phosphor bronze. Now a guitar I was thrilled
: with when I got it, doesn't make me very
: happy any more. I thought I'd ask before I
: went and changed strings again. Even my 000M
: has lost its sparkle. I don't know what I
: did wrong. I thought I could count on Martin
: strings. Was I wrong to do so? Thanks all.