I'm on lvl 45 and my mate is on lvl 31 were not able to make cubs ever after we both sleep in the den..

You have to both be a tenth level higher. For example, you both have to be level 10 to have the first cub, then level 20 to have the second, 30 to have the third, and so on. It will notify you each time you get to a high enough level to become pregnant again.

Your mate is level 31 and you're on level 45, that's the problem! You need to get your mate to level 40,(above 40 or 40, and 50-)so you can get your next baby.