First you have to get both your tamas to be in love and kissing. Then you have to 1 day for them to mate. What I always did was to get them kissing and then the next day in the morning I would connect them a couple times through out the day and then they would always mate. That's what happens to me. I'm not sure exactly what happens to you but try this and see what happens.
first of all, they have to be of opposite gender. and they have to have four hearts in the friends list of each other, they have to be partners. then if they are, they will only mate at 5yr old and up.