One of the problems is almost non-existent state support for new families. They do almost nothing for them, and then they say, well, since you're not having children, we need to bring more migrants in.
If you check birth rates of East European countries before 1989, you'll notice that the rates were pretty good back then. Although communism sucked in every imaginable way, one of the good things was that the state had your back when you had children.
Nowadays, if you decide to have children, your family expenses grow a lot and you lose a single monthly income at the same time. That's enough to bring a middle-class family into poverty. Note that in some European countries, you do receive a pittance for a limited time as a substitute for the lost income, but it's still a joke.
Putting the child(ren) into daycare and working is a no-go (at least here) for women, because the new democratic governments closed down all state-run daycares and the private ones are so expensive, it would cost you the whole income for using them.
I'm strongly right-wing, but this is something that needs to change radically. The only other option I see is denying state pension to people that had no children or a single child, because they provided no future tax payers to keep the pension system functioning.