Take apart the firing mechanism (hammer, sear, etc., and polish anything that seems to be rubbing (pins, flat surfaces, and so forth.) You'll need a Dremel and a polishing kit.  Touch up with cold blue and use some light synthetic grease on re-assembly.
The lower is glued together.  Look at the joints in the rod channel and ensure that they are smooth.  If not, use a correct diameter dowel, fine sandpaper, and sand the joints smooth.
It's possible that the hammer/sear engagement surfaces may need a bit of dressing.  I'm hesitant to give advice on this, for obvious reasons.
If you know how to do a trigger job, you will know what to look for.  If you don't, then let a professional do it.

Under no circumstances should you alter any angles/lengths of critical surfaces.   Doing so could create an unsafe condition.

And put the trigger spring back in, OK?