Other than the usual tricks I'm not sure there's much you can do. Instead of putting him in with another male, put an old shed from another male in with him and the female and leave them alone. You could also try popping another male you have and rubbing the sperm plugs on the females back. Also make sure you pair them up during weather fronts. You could also try misting them once a day. Maybe put a little Barry White on...but other than that, not sure what you can do other than continue introducing them...and hope for the best!