I bought a cup of live bloodworm, but surprisingly my 13 cm ranchu doesn't eat it, but my 10 cm ranchu likes it. How to make my 13 cm ranchu eat it?
I would say just stopping to feed anything else other than bloodworm should work after some time - hunger is the best spice; if there isn't any other food, then ultimately your fish will naturally get convinced to eat what is available. Fish need surprisingly little food to survive, but will happily eat any available excess that they could physically consume beyond the absolute minimum.
If a goldfish doesn't eat bloodworms, it means that it still has enough nutrients to go on without eating. Fish are fussy in terms of food choice only if they could "afford" it.
Don't worry please, an otherwise healthy goldfish won't die of starvation by feeding it like this. As far as I know, there are some rare species of fish that require specific food types and would starve otherwise - but goldfish are definitely not one of them. Most fish, including goldfish, are opportunistic omnivores.