Seems like up to 5% upgrade the chance is 100% so it always adds 1%. Then it changes to 70% chance so sometimes fails, but seems like it fails more often than 30% of the time.
When I've tried to upgrade a 5% weapon with 4 duplicates and a lucky stone it's telling me the success chance is 70% + 70% yet it fails all the time.
Seems like getting above a 9% weapon is not possible.
So each individual material has its own succes rate in your case your first enhance was 140% cause of the lucky stone but the second enhance was only 70% since you didnt have a second lucky stone. The best way to enhance is to use 1 material (stone/item) and enough lucky stones for 100% success. The success chance that you see when enhancing is the % of the first enhance plus lucky stone. Lucky stones only work with 1 piece of material you had 4
Put it this way. Lucky stone doubles the success %. If the success rate is 40%, add Lucky stone then it will be 80%. 1 Lucky stone : 1 material