JP and Global suggestion both.
By sacrificing only 1 unit as fodder and trying to upslot, your chances of success go down no matter what in comparison to using 2 or more units. Anything over 1 unit all has the same chance.
As an example, upslotting from 3 to 4 slots on an item is 60% with 1 fodder, but 70% with 2 or more fodders.
As someone that actually doesn't mind the affixing system in this game, this is the one major thing I have a gripe with. It just seems pointless to impose a penalty for only using 1 fodder.