You should encounter creatures in the first mine sector if they are supposed to be in that mine. If you don't, then you have encountered a bug (ironically). This always works for me when that happens...

1. Exit the mine.
2. Save your game.
3. Exit the game entirely (quit JA2).
4. Restart the game.
5. Reload the save you made in step 2.
6. Re-enter the mine.