The first thing I would do is give the trigger a GOOD cleaning. This is the procedure I use on all my C,D and E's with the Micro Motion trigger:
First I remove the trigger from rifle, and soak it for several hours in carburetor cleaner, then rinse it out well with spray brake cleaner and blow dry. I then spray all surfaces with a very light instrument oil that will not gum up. I prefer Starret Tool and Gage Oil. I then blow the trigger out again to remove all excess oil. All I'm trying to do with oil is protect metal surfaces from rust. I then reinstall trigger. Cleaning usually will take care of 95% of the trigger problems, especially if it has set for a long period of time. These guns are 50 years old now and most were never taken very good care of, especially the government guns. They gummed up from dried out oil, solvent and an accumulation of dirt. The levers then do not move freely which prevents the gun from cocking.
If someone has already messed with the adjustment screws, then you will need to readjust. Start by turning in the pull weight "Pull" adjustment screw several turns to preload levers. This is the screw furthest to the rear, and is so marked on the trigger guard. Then back off the "O.T." overtravel screw which is the next screw going forward, it is marked as well. I then move to the "Sear Engagement" screw which is the forwardmost screw at front of trigger housing. It can only be accessed with stock removed. Turn this screw CW to where engagement surfaces of the 2 levers are fully engaged. This can be seen through the "window" on RH side of trigger just to the rear of the engagement screw. You will find that when you then turn the engagement screw CCW, the lower lever will be pulled away from the upper rocker lever thus providing less engagement. I shoot for about 50% engagement of the 2 mating surfaces. Once this achieved start backing off the "Pull" screw until desired weight of pull is achieved. Don't go too light, depending on what trigger you have in your D (early D's had the C trigger while later guns used the revised D trigger which could be adjusted to a much lighter pull) After finishing with this adjustment,be sure and slam the bolt down a few times to make sure rifle does not slam fire. If it does slam fire, just increase pull until you reach a safe point. Lastly, turn "O.T." overtravel screw CW to reduce the amount of trigger shoe overtravel to the desired level. Again, make sure you do not overtighten this screw or the lower level will not have enough free movement to clear the rocker engagement surface and allow rocker to move freely. This can cause premature wear on trigger levers on the corners of the engagement surfaces. That is really all there is to it. Not nearly as complicated as it sounds once you understand how the trigger works. Here is a schematic of the trigger.