You often read that Corydoras with short whiskers will not breed. That's true if the fish are really stressed, but I have successfully bred Corys with short whiskers. In this case, I believe that the reason was genetic rather than environmental. Certainly in dealers tanks, I always look at the condition of the barbels as an indicator of overall health before I buy fish. Corydoras take a while to reach full sexual maturity, often several years, so patience is important.
Some species are also seasonal spawners, even after several generations of captive breeding.
For most species in the wild, spawning occurs in the rainy season, which corresponds to the winter period in the Northern hemisphere. This is the prime time to try for a spawning. Another thing to consider is water chemistry. For Corydoras, a change is usually the key to making things happen. This is my standard method for inducing spawning in Corydoras.