There are a number of items that can cause this. Check to see if the hammer is cocking all the way, it should move past the sear catch and rock forward coming to rest. Check if dirt landed on the hammer behind the upper sear. This can cause the sear to not seat all the way against the hammer and fall off. Remove side plates and links then run a brush on the hammer and sear and recheck. . If it is still falling off remove the trigger assembly and see if it is still falling when out of the receiver. Making sure not to change the main spring screw settings, unhook the hammer from the spring. Then tighten the large main spring screw counting the amount of turns it took to get it snug. Loosen the screw and swing the main spring out of the way and check to make sure the spring is not rubbing on the frame causing the trigger to bind. Check the small screw that is holding the trigger/sear spring to insure it is tight. If all is good you probably have a bad hammer latch and it can be reworked and reheat-treated, it would not be necessary to send the entire rifle into a smith only the assembly.