Have you been feeding her at least semi-regularly?
Has she laid eggs in the caverns? If yes, you need to collect it. (There's a glitch with the eggs, see below.)
Have you given Erevi the Niger Arcana? Should be yes, that's the trigger to break down the wall into the caverns...
Have you edited your game files, or installed any mods other than Spicy and/or Gnomes? That can bork things in the game seemingly unrelated to what the edits/mods are changing.
Venomina eggs glitch: When she lays her first eggs either one or two will appear, near where you enter the caverns. Collecting the one on the right is the trigger for continuing Venomina's story. If it's not there, go back to the dungeon & return to the caverns to have the eggs respawn (this takes no time from the clock). Repeat until the one you want appears.