The unset trigger on these is essentially the standard Mauser trigger, operated remotely by the front trigger directly levering the lobe of lobe on place of the bough. If t has become heavy it’s either a partial seized front trigger (unlikely - but easy to check, just pull the front trigger with the bottom metal separate) or more likely some issue with the sear/cocking piece, or maybe even the distance between bottom metal and receiver. Does it still fire when set? Normally this is the first to fail. Try assembling it out of the stock and check for function. If it works correctly make sure the bottom metal and receiver are the correct distance apart when in the stock - use the screw heads as a reference. Make sure the sear operates smoothly, the spring feels normal and there is no debris in the area.