Hi Mike -

There is no great answer to your question, but a few things to think about. Yes, there can be male and female trees. But, they can be monoecious as well - which is more common (male and female blooms on the same tree). If you have seen fruit before, you know you have a chance. 

My first and best educated guess is bloom time and frost. You may see flowers, but if you cut the flower open - it may be brown and dead inside. The area where you have them planted could be in a frost pocket at the time of bloom. This year, check blooms on parts of the tree to ensure you have viable blooms.

Secondly, the issue could be fertilization. Mulberry trees thrive in poor soil conditions. Here in Youngstown, they fruit quite well along streets and next to vacant homes. So, overfertilization may cause issues and encourage more growth and less concentration on fruit production. You don't need to apply manure each year. I encourage you to do a soil test to determine the pH and nutrient levels.

Here are some links for you:

https://gardeningsolutions.ifas.ufl.edu/plants/trees-and-shrubs/trees/mulberry.html - Florida - thriving in infertile soil

http://go.osu.edu/mulberry

https://naturalresources.extension.iastate.edu/forestry/iowa_trees/trees/red_mulberry.html

OK. Let us know if you need anything else! -Eric