Thanks I'm glad you're enjoying it :)
So the spawning system is based on the in game clock and tries to spawn a girl every hour, which translates to one minute of game time.
First a random number between 0 and 3 is generated, if it's not a zero, nothing happens. This means the chance to spawn a girl every minute is 25%.
Then the requirements are checked and a list of possible spawns is built, then a random type is picked from the list and that species is spawned, as long as there are less than 2 already in your garden.
I will double check that I didn't screw something up in the last update but it's probably just bad luck. A trick is to save your game a few seconds before the hour and load if nothing shows up in a few seconds (It takes new spawns about 20 seconds to move from their spawn location in the forest to your garden).