I've sat there and thrown couples together for what feels like 100 times before I had any babies conceive -

400 food mininum to keep from getting the msg -- they are too hungry for this..... need at least your first hut built before they will have enough room to grow.....

But like Barbara said persistence and patience .... just keep them embracing ----

you can always check breeding on a female you want to be nursing and get her up to Trainee Parent - then let her go - she will keep going after the men til she gets a baby (try to never set a male to breeding - you will have so many kids you wont know what to do with them)