it has nothing to do with your HR.
basically it is all to do with randomness. yup randomness ^.^. Your MHF determines that when you polish a certain amount of rustones, the 'nth' one will be an ancient weapon. basically this is what you have to do.
i suggest you get quite a couple of rustones firstly. lets say for example you get 5 ruststones.
you go to the shop and polish them all.
It turns out that none of them are an ancient weapon. You then need to reset the game and go to your previous save file
before you polished any of your stones.
After re-setting the game, you need to go and complete a couple of quests. this will reset the random numer'n'.
then attempt to polish the rustones again.
if you don't get any repeat the above until you do.