Help with problem underpainting darks

[Tito Champena]

I'm doing an underpainting with verdaccio and when it dries, certain dark parts appear to have become lighter. For example, I painted the pupils of a portrait with black and the next day it looked like I had painted a blind person with no pupils. This lighter value seems to regain its original value after I oil it or spray it with retouch varnish.

Can somebody explain to me what happened and how to avoid this problem?

Thank you