S&W New Model #3 won't fire--mainspring too short??

[DJ]

I have a recently acquired S&W NM#3 that won't fire.  It seems to have a rebounding firing pin (which I hope is normal), and just not enough "oomph" to overcome the rebounding spring (trigger spring?) and pop a primer.  When I pop the sideplate off, pull the trigger, and push the hammer all the way forward, the stirrup actually lifts slightly off of the mainspring hooks.  This seems odd, as I would think the mainspring should always remain in contact with the stirrup.  It seems that either the mainspring is a too-short replacement, or the stirrup has been replaced.  I note that there are some scratches on the interior of the frame, like someone had a hard time forcing the mainspring down to disengage it.  However, I also note that, because of its shape, the stirrup is limited in how far back it can rotate, and it looks like if it could rotate a little further, or if the spring were a tiny bit longer, it could at least stay in the mainspring hooks when the hammer was all the way forward.
So, hints anyone?  Will any of the repro mainsprings fit an original NM#3?  Should the stirrup be able to swing back freely to the bottom of the hammer?  If none of this makes sense, I could probably post a couple photos.
Thanks--
--DJ