If you can't see the papilla between the ventral and anal fin, chances are that spawning isn't being thought about with your fish. They may need to get a little bit larger still and/or they may not have found another to pair up with yet. Don't worry, when they're ready and have paired up, you will know the differences between the two and will see just how evident that make themselves (as male and female). Is there just one or a few that are as big as the biggest one...or are a majority of them the size of the silver dollars?