Is the trigger safety bar been modified? if not, that is where your problem may be.
Try cocking the hammer without levering, Did you have to slightly force the hammer to lock it's sear? if so the back of the hammer is resting on the mainspring while the mainspring is pushing against the top of the trigger safety. If so the trigger safety needs to be modified by filling some metal off the top on the area which is touching the mainspring. To see if this is happening, you need to remove the buttstock and observe the action. This situation prevents the hammer from going into lock position with the trigger sear. By modifying the trigger safety, it helps lower the mainspring therefore to let the rest of the hammer to fall into full cock. Seen this many times on 73 including mine. This happens on some 73s sometimes but not all the time.
Another way to check if this is happening is to pull the lever forward, make sure the hammer is in full cock and lock, then manually depress the safety trigger bar and check if the safety moves freely into the frame. if not, the trigger safety is pushing against the mainspring and that is what's happening as I have explained above. You have the trigger safety spring installed also, right? The safety should recoil back out when released, right?
Another option, If you don't want to modify the trigger safety, turn the mainspring screw all the way in as if to tighten then cock the hammer manually, if there is no resistance when the hammer is almost into full cock, this may solve your problem.
Just a suggestion. Your mileage may vary. Seriously, hope this help. Let us know. Good luck.