Cories are not algae eaters, they will eat some vegetable matter but they also need meaty food and they don't usually eat algae.
Otos are algae eaters, but they only eat certain types of algae.
They are wild caught, and they tend to be kept in tanks with no algae from the time they are caught to when they arrive at the store. Once they reach a certain point of starvation, they can't start eating again and usually die after they reach this point. This is why it is recommended not to buy them till they've been in the store tank for a couple of weeks as by that time, the ones that are going to die will already have done so. And buy the individual otos which have nice rounded tummies (not bloated) as those have been eating.
Many otos will eat nothing but certain types of algae, and if your tank doesn't have these types, or not enough of them for a shoal, they'll die. Some otos will start to eat commercial algae containing food, and if you can get them eating this they'll be OK.