Hungary spent 5%(!) of its GDP on direct child benefits and instituted a lifelong tax exemption for women with 3 or more children and the birth rate rose by... ~0.15 for a few years and then dropped again.
For comparison I assume you're German given the username, here this would amount to 300 billion per year, which is about 100x what the government spends on BAföG (the mentioned federal student aid). I genuinely wonder how much countries will have to spend until people realize that this has quite literally nothing to do with money.
Hungary might also have a problem with too many young Hungarian women living abroad. It is hard to raise fertility if your locally present fertile population has already tanked.
I expect it has to do with money but is much more complicated than simply allocating a small yearly amount. By way of related analogy consider housing. If you provide a subsidy but the market is primarily constrained by an inelastic supply then all you will accomplish is raising rents. I assume that there are many such nuanced factors that all contribute to the birth rate.