Hi guns,
Erik is right; your corys are just too young to spawn.
Sterbai are big, hefty corys and there with mine there is little difference in size between the males and females. While they are young, it's hard to tell them apart, but if you think you only have one male to four females, I suggest that you buy some more in hopes of getting males. Two males per female at a spawning is usually suggested to have good fertilization of all the eggs.
This does not necessarily mean that you need twice as many males as females overall. It just means that when they spawn, you should have more males than the number of females that are actually laying eggs. However, since they are schooling fish and several females might be spawning at the same time, it would be best to get a few more males.
Sterbai are not the easiest corys to get to spawn. Some experienced breeders have had trouble with them, and mine are not predictable either. This does not, by any means, mean that they are impossible to breed, just a little harder than others such as peppered corys or bronze or albino C. aeneus.